Closed-loop recycling. A case study of films for greenhouses

The degradation undergone by plastic films used for greenhouse coverage is the main cause of the worsening of the processability, of the mechanical properties and of the durability of secondary materials obtained by their recycling. This means that this secondary material cannot be processed to produce films for the same applications (closed-loop recycling).In this work different strategies are proposed to improve the processability, some mechanical properties and the resistance to photo-oxidation of these secondary materials. In particular, homopolymer blends or multilayer films where the secondary material is used in combination with virgin polymer give the best results.     

Comparative analysis of reactive extrusion of LDPE and LLDPE

Grafting of polyethylenes is an important method used in the modification of polyolefins with functional groups. Nevertheless, few systematic studies have been conducted regarding this type of reactions and the interdependent variables are not yet accurately established. In this investigation, the grafting of two commercial polyethylenes, low-density polyethylene and linear low-density polyethylene with diethyl maleate was carried out in a corotating twin-screw extruder at different extrusion conditions. In order to establish the interdependence of the extrusion conditions employed, a combined experimental and simplified one-dimensional, non-isothermal model was implemented in the study of flow in the elements of the screw-extruder. The effects of the grafting degree and the processing conditions employed in the preparation of the functionalized materials on their shear and elongational viscosities at high shear rates, and on their thermal and tensile properties were studied.     

A conjugation polyimine vitrimer: Fabrication and performance

Although polyimine vitrimers are a promising and invaluable class of self-healing materials owing to their great maneuverability and fast exchange of crosslinks without catalysts, it remains a challenge to improve the mechanical properties and thermal stability of the material by simply designing their internal structures. Herein, we report a conjugation polyimine vitrimer, which was prepared by polycondensation of terephthaldehyde, m-xylylene diamine and tris (2-aminoethyl) amine as crosslinker. The incorporation of m-xylylene diamine endows the materials with an expanded conjugation structure, which greatly improves its mechanical properties and thermal stabilities. On the other hand, m-xylylene diamine is less toxic and irritant than the linear aliphatic ones usually used, which makes the reactions more environment-friendly. Results show that these materials have excellent mechanical properties and thermal stability, perform good flexibilities after immersed in water and show very short relaxation times at elevated temperatures. In addition, especially, they exhibit good self-healing efficiency under the condition of heat, water and amine solvents without the need for any catalyst. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2531–2538     

Crystallization kinetics of virgin and processed poly(lactic acid)

Poly(lactic acid) (PLA) is an emerging material mainly because it can be synthesized from renewable resources and is thus environmentally and ecologically safe. The mechanical properties, above all the thermal resistance of PLA are determined by the crystalline content: the heat deflection temperature of crystalline PLA can reach 100 °C, whereas amorphous PLA loses mechanical properties at temperatures slightly higher than 60 °C. However, PLA has a low crystallization rate, so that after processing it remains mostly amorphous. This characteristic heavily limits the use of PLA for commercial applications. Many studies have been recently published on the crystallization kinetics of PLA. The effect of processing on this feature is however often neglected. In this work, the significance of processing on the crystallization kinetics of a commercial PLA was investigated. Two processing methods were explored: extrusion and injection moulding. The obtained materials, and the starting pellets of virgin polymer, were analyzed by calorimetry in order to obtain the crystallization kinetics. Two protocols were adopted to determine the crystallization rates during cooling from the melt or heating from the solid. The parameters of a kinetic equation were determined for all the materials and protocols adopted and it was thus possible to describe the evolution of crystallinity during heating and during cooling.     

Influence of thermophysical properties on burning behavior of intumescent fire-retardant materials

Thermophysical properties of intumescent fire-retardant (IFR) materials are important parameters as input data in modeling the combustion process of IFR materials in a fire. In this paper, the influences of several thermophysical properties on burning behavior of IFR materials are simulated based on a combustion model of IFR materials. Thermophysical properties selected here are thermal conductivity of virgin material and char layer, specific heat capacity of virgin material, density of virgin material, surface emissivity of virgin material and char layer, heat of decomposition, heat of combustion, and intumescent temperature. Predicted heat release rates curves for the IFR material at an incident heat flux of 50 kW m^?2 are shown for the varied thermophysical parameters’ values. The results show that these varied parameter values can affect the burning behavior of materials remarkably. A comparison with experimental results demonstrates that the predictions of heat release rates are in reasonably good agreement with the experiment.     

On the effectiveness of different additives and concentrations on the re-building of the molecular structure of degraded polyethylene

Mechanical recycling is an easy and economic way to re-use plastic waste as secondary materials, but, in general, their properties are worse with respect to the reclaimed materials and the virgin polymer. The aim of this work was to study the effect of concentration and reaction kinetics of two additives, an ethylene-co-glycidyl methacrylate (Lotader) and a hydroxylamine derivative (CGX), in the re-building of a degraded polyethylene. CGX is a nitroxyl radical generator able to form branching in polyolefins while the epoxy groups of Lotader can react with the functional groups present in the recycled polyethylene. The results indicate that the CGX has a higher reaction rate than Lotader, probably due to its lower molecular weight and the different reaction path. As for the effect of concentration, as expected, a higher amount of additive accelerates and increases re-building, especially when CGX is used. The melt strength increases with the additive content and the mechanical properties show a significant reduction at the highest concentrations of both additives due to excessive cross-linking.     

Monitoring abiotic degradation of branched polyethylenes formulated with pro-oxidants through different mechanical tests

The mechanical properties of two linear low density and low density polyethylenes containing a pro-oxidant additive were monitored during accelerated aging (60 °C in a convection oven) and weather exposure. Tearing tests (trouser) were performed for the first time in polyethylenes subjected to oxo-degradation revealing a transition from an extensible to a non-extensible material, at exposure times when standard tensile tests were not able to detect any changes in the materials. The essential work of fracture (EWF) technique was also applied and the results were in agreement with those of trouser tests. The specific essential work of fracture first increased with exposure time until the sample experienced a transition to a less ductile state where EWF was no longer applicable. EWF and trouser tear tests were more sensitive detecting the onset of degradation probably because they employ notched specimens that impose more critical stress concentration conditions than conventional tensile tests.     

Thermoreversible covalent crosslinking of maleated ethylene/propylene copolymers with diols

The objective of this study is the thermoreversible crosslinking of maleated ethylene/propylene copolymer (MAn‐g‐EPM) using the equilibrium reaction with diols. Covalent hemi‐ester crosslinks are formed via the reaction of anhydrides with alcohols, while an equilibrium shift at elevated temperatures may result in their removal. High conversions to hemi‐ester are obtained at low temperatures in the presence of p‐toluenesulfonic acid, whereas conversions are low at high temperatures. The presence of microphase‐separated aggregates acting as physical crosslinks was demonstrated for MAn‐g‐EPM and all crosslinked materials. The covalent crosslinks were only formed within the aggregates, resulting in stronger aggregates that persisted to higher temperatures. The tensile strength and elasticity were significantly improved upon increasing level of crosslinking, whereas the type of diol has less influence. The covalently crosslinked MAn‐g‐EPM was reprocessable via compression molding at temperatures above 175 °C. Irreversible diester formation occurred for the longer diols, but did not prevent reprocessing, while short diols evaporated. Both effects lowered the level of crosslinking, resulting in significantly changed mechanical properties. The reprocessability does not originate from an equilibrium shift, but from a dynamic exchange between crosslinked and non‐crosslinked functional groups, which allows crosslinks to disconnect and the corresponding chain segments to diffuse between aggregates. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1810–1825, 2008     

Natural weathering of low density polyethylene: Part 2—Dielectric properties

Dielectric properties of naturally weathered low density polyethylenes have been measured as a function of temperature, frequency and weathering time. The results show that dielectric loss, dielectric constant and the β transition increase with the weathering time. However, the deterioration of the dielectric properties is strongly reduced by the presence of a uv stabilizer system.The changes in the dielectric properties can be qualitatively explained by the formation of polar groups and crosslinks.     

Bulk plastic materials obtained from processing raw powders of renewable natural polymers via back pressure equal channel angular consolidation (BP-ECAC)

Renewable natural polymers wheat starch (WS) and wheat gluten (WG) were successfully processed into plastic bulk materials using back pressure equal channel angular consolidation (BP-ECAC) without using any additional plasticizers at relatively low temperatures. The strong shear deformation occurred during the process caused an effective deformation of WS or WG granular structures and resulted in an efficient gelatinization of starch or plasticization of gluten with the natural moisture content. Sufficient chain entanglement was formed in both WS and WG materials for achieving strong cohesion among the macromolecule matrixes. The mechanical strength of the obtained plastic materials was comparable to that of conventional polymers but stronger than the strength of thermoplastic WS or plasticized WG. The processing temperature played an important role in determination of morphologies and properties of the plastic materials. Increasing processing temperature would cause more effective gelatinization or plasticization of the natural polymers, enhance the interactions among different components in the systems, and form materials with improved mechanical properties. Thermal cross-linking might play a positive role in the improvement of mechanical properties when processing temperature was increased. However, thermal decomposition could also occur under such severe shearing especially at high temperatures. The optimum temperature for conducting such process was around 100-120 °C for WS and WG. The BP-ECAC method provides a potential to manufacture natural polymer based plastic materials efficiently on an industrial scale for various applications.     

Dopamine Self-Polymerization as a Simple and Powerful Tool to Modulate the Viscoelastic Mechanical Properties of Peptide-Based Gels

Dopamine is a small versatile molecule used for various biotechnological and biomedical applications. This neurotransmitter, in addition to its biological role, can undergo oxidative self-polymerization to yield polydopamine, a robust universal coating material. Herein, we harness dopamine self-polymerization to modulate the viscoelastic mechanical properties of peptide-based gels, expanding their ever-growing application potential. By combining rapid peptide assembly with slower dopamine auto-polymerization, a double network gel is formed, where the fibrillar peptide gel network serves as a scaffold for polydopamine deposition, allowing polydopamine to interpenetrate the gel network as well as establishing crosslinks within the matrix. We have shown that triggering the assembly of a lysine-rich peptide gelator in the presence of dopamine can increase the mechanical rigidity of the resultant gel by a factor of 90 in some cases, while retaining the gel’s shear thin-recovery behavior. We further investigate how factors such as polymerization time, dopamine concentration and peptide concentration alter the mechanical properties of the resultant gel. The hybrid peptide–dopamine gel systems were characterized using rheological measurements, circular dichroism spectroscopy and transmission electron microscopy. Overall, triggering peptide gelation in the presence of dopamine represents a simple yet powerful approach to modulate the viscoelastic mechanical properties of peptide-based gels.     

Degradation of Mater-Bi/wood flour biocomposites in active sewage sludge

The mechanical properties of Mater-Bi^® are, in general, not adequate for certain applications and the addition of a filler is therefore necessary. Among the different fillers, natural fibres are particularly interesting because they potentially allow improving the performance of the material without compromising its biodegradability.In order to improve the basic mechanical properties of a Mater-Bi grade and to obtain a new, fully biodegradable material, wood flour based composites were prepared by different processing methods. To simulate actual and not laboratory bacterial attack on the prepared materials, in this work we studied the biodegradation of the composites in a real active sewage sludge reactor. In particular, the biodegradation rates were investigated with reference to different pre-treatments of the materials and different environmental conditions (summer and winter). The results showed that wood flour enhances the biodegradability of the materials. The results indicated also strong relationships between the surface roughness and the biodegradation rates (in particular, higher roughness leads to wider bacterial attack). The different processing techniques had direct effects on the overall biodegradation rates. In particular, when higher smoothness and packing is achieved, the biodegradation rate is lower. The mechanical analysis indicated that adding wood flour to Mater-Bi has positive effects on the elastic modulus, but when the bacterial attack becomes critical, a general sudden drop of the mechanical properties is observed.     

Morphology and strain rate effects on heat generation during the plastic deformation of polyethylene/carbon black nanocomposites

The phenomenon of internal heat generation during the plastic deformation of polyethylene/carbon black nanocomposites at high strain rates was investigated using a high resolution thermal camera. Material morphology, strain rate and carbon black (CB) content were found to be critical factors that affected heat generation during tensile testing, and consequently changed the mechanical behaviour. Two processing methods (M1 and M2) were used to prepare the materials, with CB contents of 0.5, 1 and 3 wt.%. The results showed a significant increase in internal heat generation after yielding, with temperatures exceeding 70 °C for materials processed using M1 and 55 °C for materials processed using M2. The temperature increase was dependent on the processing method, the CB content and the strain rate. The increase in temperature due to plastic heat generation affected the properties of the material, reducing the plastic hardening and reducing the tensile strength at high strain rates. This is of significance when considering the use of these materials in applications involving high strain rates, such as impact protection.     

Comparison of the transient stress–strain response of rubber to its linear dynamic behavior

Master curves of the small strain and dynamic shear modulus are compared with the transient mechanical response of rubbers stretched at ambient temperature over a seven‐decade range of strain rates (10^?4 to 10³ s^?1). The experiments were carried out on 1,4‐ and 1,2‐polybutadienes and a styrene–butadiene copolymer. These rubbers have respective glass transition temperatures, T_g, equal to ?93.0, 0.5, and 4.1 °C, so that the room temperature measurements probed the rubbery plateau, the glass transition zone, and the onset of the glassy state. For the 1,4‐polybutadiene, in accord with previous results, strain and strain rate effects were decoupled (additive). For the other two materials, encroachment of the segmental dynamics precluded separation of the effects of strain and rate. These results show that for rubbery polymers near T_g the use of linear dynamic data to predict stresses, strain energies, and other mechanical properties at higher strain rates entails large error. For example, the strain rate associated with an upturn in the modulus due to onset of the glass transition was three orders of magnitude higher for large tensile strains than for linear oscillatory shear strains. © 2011 Wiley Periodicals, Inc.* J Polym Sci Part B: Polym Phys, 2011     

层间水含量对Mg3Al-LDHs-Cl力学特性的影响

构建水滑石(Mg3Al-LDHs-Cl-nH2O)周期性计算模型, 采用密度泛函理论-赝势平面波法的CASTEP/LDA(局域密度近似)程序, 在CA-PZ水平上对模型进行几何全优化和弹性常数计算. 从弹性常数Cij、切变模量、杨氏模量和泊松比等方面研究了层间水分子含量(n)对材料力学性质的影响. 计算结果表明, 层间水分子含量对材料的力学性质有很大的影响. 层间水分子能提高体系总体的抗压性能, 当n=1时, 材料的抗压性能最好; 当n=2时, 材料的抵抗剪切变形的能力最差, 体系最柔软. 层间水分子对材料杨氏模量的影响较大, 对泊松比的影响并不明显. 且层间水分子对材料横向的力学性能起到平均化作用, 最终使材料在x轴和y轴方向上的抗压性能和膨胀率趋于一致.     

A method for more accurate FEA results on a medical device developed by 3D technologies

At present, 3‐dimensional models of all additive manufactured objects (AMOs) are accepted as a solid model for finite element analysis (FEA). FEA of AMOs may not reveal the real results because mechanical properties of default materials in CAD software and newly built AMOs are not equal to each other. This may produce problems especially for the end user due to unexpected failure or wear off. The aim of this study was to compare FEA results of an additive manufactured Ankle Foot Orthosis model under 2 different value sets, namely default material‐based mechanical properties and measured mechanical properties. In order to determine the real mechanical properties of the additive manufactured Ankle Foot Orthosis, 3‐dimensional printed test specimens with different infill densities were prepared and tested according to the recommended standards. Mechanical test results were then loaded in the CAD software and FEA was performed. This study illustrated that default mechanical properties of existing materials in CAD software produce misleading simulation results for AMOs, ie, real mechanical properties should be used to get more accurate results.     

Structural effects on thermal properties and morphology in XLPE

This study investigated the morphological, thermal and mechanical changes with increasing crosslink density for two low density polyethylenes (LDPE). A reference LDPE was compared with an LDPE containing a higher number of vinyl groups that was introduced via a copolymerisation with a diene. During crosslinking, two reactions simultaneously occur in the copolymer, i.e. a reaction of the vinyl groups and combination crosslinking. After crosslinking with a low amount of peroxide, the majority of the crosslinks originate from reacted vinyl groups in the LDPE containing the higher number of vinyl groups, whereas the crosslinks in the reference LDPE originate from combination crosslinking, thus leading to different crosslinked structures for the two polymers. The melt temperature, crystallisation temperature, and degree of crystallinity were measured using a Differential Scanning Calorimeter. Thermal fractionation studies and morphology studies were also made. The Differential Scanning Calorimetry results show a decrease in those properties for both materials along with a concurrent change in the morphology when the crosslink density increased. The results deviate slightly between the materials.     

Void formation due to gas evolution during the recycling of Acrylonitrile-Butadiene-Styrene copolymer (ABS) from waste electrical and electronic equipment (WEEE)

During processing of recycled ABS and ABS/HIPS blends, voiding defects can occur within the resulting material which can result in deterioration of mechanical properties. The voids were previously thought to be caused by the evolution of volatile substances during processing. This study investigated the recycling of post-consumer ABS from a variety of types of WEEE. The mechanical properties of the processed material were assessed and a combination of visual observation during processing and optical microscopy was used to identify the extent of voiding. It was found that flexural strength and ductility in particular decreased with increased levels of voids. The gases emitted during heating and processing were analysed using Gas Chromatography with Mass Spectroscopy (GCMS) and were found to be breakdown products of the original polymers. These seem to be present in the WEEE, either as polymerization residuals or as products of degradation during the initial service life rather than degradation products from reprocessing. The amounts of volatiles liberated were quantified, which showed that the volatile emissions from post-consumer material were of a similar magnitude to those seen with virgin material. More intensive or longer processing led to a reduction in the emissions and voiding and an improvement in strength, suggesting that there is a finite potential for volatile liberation, and that the problem could be overcome by the use of suitable processing conditions.     

Properties of crosslinked polylactides (PLLA & PDLA) by radiation and its biodegradability

Crosslinked materials derived from poly(lactide) (PLA) have been produced by radiation modification in the presence of a suitable crosslinker (triallyl isocyanurate) (TAIC). The crosslinking structure introduced in PLA films has not only much improved the heat stability but also their mechanical properties. The properties of crosslinked samples are governed by crosslinking density and these improvement seemed to increase with radiation dose. This implied that the three dimensional networks have been introduced in material by radiation and the crosslinking density depended on the structure and length of PLA chains. Biodegradability of PLA was also determined by an enzymatic degradation test and burying in compost at 55 °C. Differing with PLLA, PDLA was insignificantly degraded by proteinase K. The degradation rate of PLA in compost was postponed with the introduction of crosslinks.     

Mechanical behavior of injection-molded starch-based polymers

This work evaluates the mechanical performance of injection-molded starch-based copolymers, 60/40 (mol/mol) starch/poly(ethylene vinyl alcohol), and the possibility of improving material properties through deliberately induced anisotropy during processing. Different types of samples were produced by conventional and shear-controlled injection molding (Scorim) and tested under tensile and impact loading. The behavior of three distinct grades is discussed in terms of the respective fracture morphology (evaluated by scanning electron microscopy). A comparison is made between the behavior of conventional and Scorim samples. The results show that the mechanical properties of the materials used were significantly improved by the employment of the Scorim process. The stiffness values of the conventional moldings were doubled, without reducing the ductility of the polymer. The impact data showed a material sensitivity, and consequent loss of properties, to the localized shear imposed to the melt during processing. This situation is attributed to very narrow mold gates (in the case of pingated systems) and leads to much reduced impact performance.