As biodegradable thermoplastics are more and more penetrating the market of filaments for fused deposition modeling (FDM) 3D printing, fillers in the form of natural fibers are convenient: They have the clear advantage of reducing cost, yet retaining the filament biodegradability characteristics. In plastics that are processed through standard techniques (e.g., extrusion or injection molding), natural fibers have a mild reinforcing function, improving stiffness and strength, it is thus interesting to evaluate whether the same holds true also in the case of FDM produced components. The results analyzed in this review show that the mechanical properties of the most common materials, i.e., acrylonitrile-butadiene-styrene (ABS) and PLA, do not benefit from biofillers, while other less widely used polymers, such as the polyolefins, are found to become more performant. Much research has been devoted to studying the effect of additive formulation and processing parameters on the mechanical properties of biofilled 3D printed specimens. The results look promising due to the relevant number of articles published in this field in the last few years. This notwithstanding, not all aspects have been explored and more could potentially be obtained through modifications of the usual FDM techniques and the devices that have been used so far.

Objective: To investigate the stress release properties of four thermoplastic materials used to make orthodontic aligners when subjected to 24 consecutive hours of deflection. Materials and Methods: Four types of aligner materials (two single and two double layered) were selected. After initial yield strength testing to characterize the materials, each sample was subjected to a constant load for 24 hours in a moist, temperature-regulated environment, and the stress release over time was measured. The test was performed three times on each type of material.

The aim of this article is to investigate the influence of filler content and temperature on the rheological, mechanical, and thermal properties of wood flour polypropylene composites (WPCs). Testing WPCs at high temperatures and percentages of filler is extremely challenging because of reduced linear viscoelastic region, high viscosity, and degradation. In this work, a complete characterization of WPCs with different filler percentages (0–70 wt%) has been made. Rheological tests are performed at 170°C for the WPCs and in the 170–200°C range for neat polypropylene. A single master curve is obtained using two shift factors that can be described by a modified Eilers model and a Williams‐Landel‐Ferry equation. This master curve, fitted with a Carreau‐Yasuda model, can be very useful for predicting the viscosity of WPCs at temperatures that are typically used during processing and for any percentage of filler. POLYM. COMPOS., 37:3460–3473, 2016. © 2015 Society of Plastics Engineers

Green composites, i.e. biodegradable polymers reinforced with natural fibers, are attracting interest as potential substitutes for conventional composites based on petroleum derived plastics. The role of the inherently complex morphology of natural fibers in their reinforcing mechanisms is not completely understood and this is the topic of the present study. The selected system was poly-(lactic acid) filled with 3 and 6 wt.% of short hemp fibers. Such a low fiber amount was chosen to help visualization of the fibermatrix interface at the scanning electron microscope. Remarkable differences in the mechanical behavior were found between composites containing fibers that were alkali treated with respect to untreated fiber filled materials, but unexpectedly it was found that the quality of the fibermatrix interface was only marginally influenced by the alkaline treatment. Interface properties were thus not exhaustive in explaining the observed differences. On the other hand, the main difference between treated and untreated fibers was the presence, in the untreated fibers population, of a volumetrically relevant sub-population of thick fiber bundles. It was further argued that this fraction did not carry the loads transferred across the fiber-matrix interface uniformly in its cross section, thus determining a reduction in the effective fiber volume fraction. In contrast, the combined action of alkalization and the mechanical stresses during melt mixing resulted in a narrow distribution of isolated elementary fibers, which were more effective in providing higher mechanical properties, in agreement with theoretical predictions. The key message for the scientific community interested in maximizing the mechanical performances of green composites is that, besides trying to improve the quality of the fiber-matrix interface, one should also aim at minimizing the amount of fiber bundles.

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Wood polymer composites are materials frequently used for outdoor applications, since they can operate under a large range of temperature and moisture conditions. The present work investigates the combined effect of water soaking and temperature (ranging from 10 up to 40 °C) on wood flour filled high density polyethylene. Water soaking tests have revealed that the percentage of absorbed water at the saturation plateau increases with temperature due to the induced stiffness reduction of the polymeric matrix. This hypothesis has been confirmed by scanning electron microscopy analyses, which has allowed to observe the filler–matrix interface and to measure the wood flour particle size. In addition, instrumented Charpy impact tests have shown a strong toughness reduction as a consequence of fiber–matrix debonding induced by particle swelling, which worsens with temperature. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46674.

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Despite the fact that wood polymer composites are interesting materials for many different reasons, they are quite difficult to shape through standard polymer processing techniques, such as extrusion or injection molding. Rheological characterization can be very helpful for understanding the role played by the many variables that are involved in manufacturing and to achieve a good quality final product through an optimized mix of formulation and processing parameters. The main methods that have been used for the rheological characterization of these materials are capillary and parallel plate rheometry. Both are very useful: rotational rheometry is particularly convenient to investigate the compounding phase and obtain structural information on the material, while capillary viscometry is well suited to understand final manufacturing. The results available in the literature at the moment are indeed very interesting and are mostly aimed at investigating the influence of the material formulation, the additives in particular, on the structural, mechanical, and morphological properties of the composite: despite a good number of papers, though, it is difficult to draw general conclusions, as many issues are still debated. The purpose of this article was to overview the state of the art and to highlight the issues that deserve further investigation.