Clemens Holzer

Kinetics and mechanism of the biodegradation of PLA/clay nanocomposites during thermophilic phase of composting process

The degradation mechanism and kinetics of polylactic acid (PLA) nanocomposite films, containing various commercially available native or organo-modified montmorillonites (MMT) prepared by melt blending, were studied under composting conditions in thermophilic phase of process and during abiotic hydrolysis and compared to the pure polymer. Described first order kinetic models were applied on the data from individual experiments by using non-linear regression procedures to calculate parameters characterizing aerobic composting and abiotic hydrolysis, such as carbon mineralization, hydrolysis rate constants and the length of lag phase. The study showed that the addition of nanoclay enhanced the biodegradation of PLA nanocomposites under composting conditions, when compared with pure PLA, particularly by shortening the lag phase at the beginning of the process. Whereas the lag phase of pure PLA was observed within 27days, the onset of CO2 evolution for PLA with native MMT was detected after just 20days, and from 13 to 16days for PLA with organo-modified MMT. Similarly, the hydrolysis rate constants determined tended to be higher for PLA with organo-modified MMT, particularly for the sample PLA-10A with fastest degradation, in comparison with pure PLA. The acceleration of chain scission in PLA with nanoclays was confirmed by determining the resultant rate constants for the hydrolytical chain scission. The critical molecular weight for the hydrolysis of PLA was observed to be higher than the critical molecular weight for onset of PLA mineralization, suggesting that PLA chains must be further shortened so as to be assimilated by microorganisms. In conclusion, MMT fillers do not represent an obstacle to acceptance of the investigated materials in composting facilities.

A Review of Multiscale Computational Methods in Polymeric Materials

Polymeric materials display distinguished characteristics which stem from the interplay of phenomena at various length and time scales. Further development of polymer systems critically relies on a comprehensive understanding of the fundamentals of their hierarchical structure and behaviors. As such, the inherent multiscale nature of polymer systems is only reflected by a multiscale analysis which accounts for all important mechanisms. Since multiscale modelling is a rapidly growing multidisciplinary field, the emerging possibilities and challenges can be of a truly diverse nature. The present review attempts to provide a rather comprehensive overview of the recent developments in the field of multiscale modelling and simulation of polymeric materials. In order to understand the characteristics of the building blocks of multiscale methods, first a brief review of some significant computational methods at individual length and time scales is provided. These methods cover quantum mechanical scale, atomistic domain (Monte Carlo and molecular dynamics), mesoscopic scale (Brownian dynamics, dissipative particle dynamics, and lattice Boltzmann method), and finally macroscopic realm (finite element and volume methods). Afterwards, different prescriptions to envelope these methods in a multiscale strategy are discussed in details. Sequential, concurrent, and adaptive resolution schemes are presented along with the latest updates and ongoing challenges in research. In sequential methods, various systematic coarse-graining and backmapping approaches are addressed. For the concurrent strategy, we aimed to introduce the fundamentals and significant methods including the handshaking concept, energy-based, and force-based coupling approaches. Although such methods are very popular in metals and carbon nanomaterials, their use in polymeric materials is still limited. We have illustrated their applications in polymer science by several examples hoping for raising attention towards the existing possibilities. The relatively new adaptive resolution schemes are then covered including their advantages and shortcomings. Finally, some novel ideas in order to extend the reaches of atomistic techniques are reviewed. We conclude the review by outlining the existing challenges and possibilities for future research.

Strategies to improve the mechanical properties of high-density polylactic acid foams

In this study, different strategies to improve the mechanical properties of physically foamed high-density polylactic acid sheets were examined to produce polylactic acid foam sheets with tailor-made mechanical properties. The first part was the determination of the effect of different blowing agents (CO2 and N2) on the foam morphology. The second part of the study was the modification of the formulation. For this purpose, both a linear and a branching chain extender and a thermoplastic elastomer were used to improve the elongational properties (tensile modulus and strain at break) of the polylactic acid foam sheets. Additionally, the effect of the addition of cellulose fibers on the foam morphology and the mechanical properties was investigated. All experiments were carried out on a laboratory flat-film line. This extrusion line consists of a 30-mm single-screw extruder attached with a 250-mm flat sheet die. The results show a strong influence of the material formulation on the mechanical properties of the high-density foam sheets. Both the mechanical properties and foam morphology could be improved by the right material formulation. The addition of the thermoplastic elastomer leads to a better foam morphology and also to a reduced brittleness of the foam sheets. Furthermore, it could be demonstrated that cellulose fiber can be used as a nucleating agent for polylactic acid but causes a further decrease in the strain at break.

Atomistic Modelling of Confined Polypropylene Chains between Ferric Oxide Substrates at Melt Temperature

The interactions and conformational characteristics of confined molten polypropylene (PP) chains between ferric oxide (Fe2O3) substrates were investigated by molecular dynamics (MD) simulations. A comparative analysis of the adsorbed amount shows strong adsorption of the chains on the high-energy surface of Fe2O3. Local structures formed in the polymer film were studied utilizing density profiles, orientation of bonds, and end-to-end distance of chains. At interfacial regions, the backbone carbon-carbon bonds of the chains preferably orient in the direction parallel to the surface while the carbon-carbon bonds with the side groups show a slight tendency to orient normal to the surface. Based on the conformation tensor data, the chains are compressed in the normal direction to the substrates in the interfacial regions while they tend to flatten in parallel planes with respect to the surfaces. The orientation of the bonds as well as the overall flattening of the chains in planes parallel to the solid surfaces are almost identical to that of the unconfined PP chains. Also, the local pressure tensor is anisotropic closer to the solid surfaces of Fe2O3 indicating the influence of the confinement on the buildup imbalance of normal and tangential pressures.

Effect of the printing bed temperature on the adhesion of parts produced by fused filament fabrication

ABSTRACT For parts produced by fused filament fabrication (FFF) the adhesion between the first printed layer and the printing bed is crucial, since it provides the foundation to the subsequent layers. Inadequate adhesion can result in poor printing quality or destroyed bed surfaces. This study aims at understanding and optimising the adhesion process for parts produced by FFF. The consequences of varying printing bed temperatures on the adhesion of two commonly used printing materials on two standard bed surfaces were investigated by means of an in-house built adhesion measurement device and complemented by contact angle measurements. This study shows a significant increase in adhesion forces, when printing parts at a bed temperature slightly above the glass transition temperature of the printing material.

Optimisation of the Adhesion of Polypropylene-Based Materials during Extrusion-Based Additive Manufacturing

Polypropylene (PP) parts produced by means of extrusion-based additive manufacturing, also known as fused filament fabrication, are prone to detaching from the build platform due to their strong tendency to shrink and warp. Apart from incorporating high volume fractions of fillers, one approach to mitigate this issue is to improve the adhesion between the first deposited layer and the build platform. However, a major challenge for PP is the lack of adhesion on standard platform materials, as well as a high risk of welding on PP-based platform materials. This study reports the material selection of build platform alternatives based on contact angle measurements. The adhesion forces, investigated by shear-off measurements, between PP-based filaments and the most promising platform material, an ultra-high-molecular-weight polyethylene (UHMW-PE), were optimised by a thorough parametric study. Higher adhesion forces were measured by increasing the platform and extrusion temperatures, increasing the flow rate and decreasing the thickness of the first layer. Apart from changes in printer settings, an increased surface roughness of the UHMW-PE platform led to a sufficient, weld-free adhesion for large-area parts of PP-based filaments, due to improved wetting, mechanical interlockings, and an increased surface area between the two materials in contact.

Additive Manufacturing of Metallic and Ceramic Components by the Material Extrusion of Highly-Filled Polymers: A Review and Future Perspectives

Additive manufacturing (AM) is the fabrication of real three-dimensional objects from metals, ceramics, or plastics by adding material, usually as layers. There are several variants of AM; among them material extrusion (ME) is one of the most versatile and widely used. In MEAM, molten or viscous materials are pushed through an orifice and are selectively deposited as strands to form stacked layers and subsequently a three-dimensional object. The commonly used materials for MEAM are thermoplastic polymers and particulate composites; however, recently innovative formulations of highly-filled polymers (HP) with metals or ceramics have also been made available. MEAM with HP is an indirect process, which uses sacrificial polymeric binders to shape metallic and ceramic components. After removing the binder, the powder particles are fused together in a conventional sintering step. In this review the different types of MEAM techniques and relevant industrial approaches for the fabrication of metallic and ceramic components are described. The composition of certain HP binder systems and powders are presented; the methods of compounding and filament making HP are explained; the stages of shaping, debinding, and sintering are discussed; and finally a comparison of the parts produced via MEAM-HP with those produced via other manufacturing techniques is presented.

Rheological properties of wood polymer composites and their role in extrusion

The influence of the rheological behaviour of PP based wood plastic composites (WPC) has been investigated in this research by means of a high pressure capillary rheometer incorporating dies having different geometries. The rheological experiments were performed using slit and round dies. The influence of moisture content on the flow properties of the WPC has been investigated as well. It was observed that higher moisture contents lead to wall slippage effect. Furthermore, measured viscosity data have been used in flow simulation of an extrusion profile die. Also, the influence of different rheological models on the simulation results is demonstrated. This research work presents a theoretical and experimental study on the measurement and prediction of the die pressure in the extrusion process of wood-plastic composite (WPC).

Influence of Processing Conditions on the Morphology of Expanded Perlite/Polypropylene Composites

Perlite is an oversaturated, volcanic, glassy rock, which has chemically bound water from 2 to 5 wt%. Upon heating, perlite can be expanded up to 20 times of its original volume. Important applications are in the field of building industry, in refrigeration engineering or the pharmaceutical industry. As mineral filler in polymers, expanded perlite can increase the thermal conductivity, the viscosity and the mechanical properties of polypropylene composites. But there are still many challenges that must be analyzed to reach the full potential of those composites. This research work focuses on the morphology of expanded perlite/polypropylene (PP) compounds and the interactions between filler and polymer. To achieve good performance a homogenous dispersion of the filler in the polymer matrix is needed because the enhancement of the material correlates strongly with the morphology of the composite. Therefore it is necessary to characterize the microstructure of these materials in order to establish adequate s...

Effect of Particle Size on the Properties of Highly-Filled Polymers for Fused Filament Fabrication

Fused Filament Fabrication (FFF) could replace injection molding as the shaping step in a process similar to powder injection molding (PIM). Herein after shaping by using a highly-filled polymer the part is debound and sintered to obtain a solid part of metal or ceramic. New feedstock materials have been developed that can be printed using conventional FFF equipment. And after debinding and sintering stainless steel parts can be obtained. However, there are many parameters that can affect the performance of the FFF feedstock materials. One important parameter is the particle size distribution of the filler particles. In this paper, feedstocks containing 316L steel powder with different particle size distributions were characterized in terms of viscosity and mechanical properties, and tested regarding the printability using a conventional FFF machine. It has been observed that the particle size significantly affects the properties of feedstock materials and thus their ability to be printed.Fused Filament Fabrication (FFF) could replace injection molding as the shaping step in a process similar to powder injection molding (PIM). Herein after shaping by using a highly-filled polymer the part is debound and sintered to obtain a solid part of metal or ceramic. New feedstock materials have been developed that can be printed using conventional FFF equipment. And after debinding and sintering stainless steel parts can be obtained. However, there are many parameters that can affect the performance of the FFF feedstock materials. One important parameter is the particle size distribution of the filler particles. In this paper, feedstocks containing 316L steel powder with different particle size distributions were characterized in terms of viscosity and mechanical properties, and tested regarding the printability using a conventional FFF machine. It has been observed that the particle size significantly affects the properties of feedstock materials and thus their ability to be printed.