Valentina Mazzanti

Stress relaxation properties of four orthodontic aligner materials: A 24-hour in vitro study

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. RESULTS All polymers analyzed released a significant amount of stress during the 24-hour period. Stress release was greater during the first 8 hours, reaching a plateau that generally remained constant. The single-layer materials, F22 Aligner polyurethane (Sweden & Martina, Due Carrare, Padova, Italy) and Duran polyethylene terephthalate glycol-modified (SCHEU, Iserlohn, Germany), exhibited the greatest values for both absolute stress and stress decay speed. The double-layer materials, Erkoloc-Pro (Erkodent, Pfalzgrafenweiler, Germany) and Durasoft (SCHEU), exhibited very constant stress release, but at absolute values up to four times lower than the single-layer samples tested. CONCLUSIONS Orthodontic aligner performance is strongly influenced by the material of their construction. Stress release, which may exceed 50% of the initial stress value in the early hours of wear, may cause significant changes in the behavior of the polymers at 24 hours from the application of orthodontic loads, which may influence programmed tooth movement.

In – line vs. off – line rheological characterization of wood polymer composites

In a previous study [1], a commercial PP - based Wood Polymer Composite (WPC) filled with various percentage of white fir fibers (30% – 70% wt.) has been investigated with a parallel plate rheometer in oscillation mode at 170 °C. This temperature has been imposed by the requirement of performing the rheological test within the linear viscoelasticity region (LVR), but the data that are measured are not directly useful for processing, as the required temperature lies between 180 – 195 °C. In order to obtain the WPC viscosity at such temperatures, a model that uses the WPC viscosity measured at 170 °C at different fibers quantity and of neat polypropylene viscosity measured at various temperatures has been proposed. These measurements allowed to obtain shift factors useful to estimate the WPC viscosity at any temperature and percentage of fibers.The aim of this work has been to validate such a model using an in – line rheometer directly connected to a single screw extruder. With this apparatus, 30% and 70% w...

Rheological and thermal properties of PP-based WPC

Wood Plastic Composite (WPC) has attracted great interest in outdoor building products for the reduced cost and the possibility of using recycled materials. Nevertheless the material shows two problems: the large viscosity due to the presence of high concentrations of filler and the degradation of cellulose during processing The aim of this work was to investigate the rheological and thermal properties of WPC. The material used for the experiments was a commercial PP-based WPC compound, with different concentrations of natural fibers (30, 50, 70% wt.). The thermal properties were studied to check for degradation of natural fibers during the subsequent rheological tests. Analyzing the storage and loss moduli and the complex viscosity curves obtained using a parallel plate rheometer it was possible to observe some features related to the viscoelastic nature of the composite.