1,721,067 research outputs found
Cephalometry of cleft lip and palate
No full textThis study demonstrated that there were basic differences in facial morphology between patients with cleft lip and palate and subjects with non clefting. The findings are obtained on cephalometric analysis of skull's teleradiograph L-L. These differences are for all pratical purposes confined to the maxillary complex. The mandible appear, instead, normal unless in the position in every respect in the not cleft lip and palate subjects
A dive into rubber: structure-property correlation
Full text linkAn extended characterization of several compounds, differing in the nature of the rubbery matrix (Natural rubber, Butadiene Rubber, Isoprene Rubber) as well in the type and content of the filler adopted (carbon black, silica), have been characterized under different loading conditions (uniaxial, biaxial and pure shear loading conditions) from the small strains (DMA) up to fracture. A correlation between the structural features of the compound and its mechanical behaviour has been proposed for the different materials under consideration
Stretch-induced softening in filled elastomers: A review on Mullins effect related anisotropy and thermally induced recovery
No full textThe Mullins effect of filled elastomers, corresponding to their softening under stretching, has been widely
explored and several models have been proposed to reproduce it. However, the observed anisotropy in the
material mechanical response induced by a stretch and the thermally induced recovery of the pristine material
behaviour are two aspects of the phenomenon that are difficult to model. In this review, an update collection of
the main results from literature experimental works about these topics is reported, with the aim to provide a
support for the improvement of existing models or the formulation and validation of new ones. A part of the
review details testing methods used to investigate the anisotropic mechanical response induced by differently
pre-stretching the materials. Another part, focused on studies highlighting the recovery of the original behavior
after a thermal treatment of the stretch-softened material, is aimed at approaching a more comprehensive understanding of the phenomena. The results of literature studies are summarized emphasizing the type of rubber
and filler, the filler content as well the applied stretching conditions. An overview of physical interpretations of
stretch-induced anisotropy and thermal recovery is provided, which could be useful for the development of the
physically based models
EFFECT OF CROSSLINKING PROCESS PARAMETERS ON THE RHEOLOGICAL BEHAVIOR OF A POLYDIMETHYLSILOXANE
No full textHumidity responsive bi-layered composites
No full textAmong cellulose-based materials, which are known for their hygroscopic behaviour, cellulose acetate (53.3 % of acetylation) has been chosen for this investigation. Membranes of cellulose acetate have been prepared via solvent evaporation of cellulose acetate and ethyl lactate solutions. Membranes with thicknesses within the range of 66÷200 μm have been manufactured. Via gravimetric measurement of preliminary dried membranes, the moisture absorption at room temperature and different relative humidity (RH = 21÷53%) have been monitored. A moisture diffusion coefficient of 3.35×10-6 mm2⁄s and a relaxation factor of 0.026 s-1 have been determined. A dependency of moisture concentration at saturation on relative humidity has been observed. The induced expansion due to the moisture absorption has been determined by thermomechanical analysis. The hygroscopic expansion coefficient of cellulose acetate has been evaluated as 192.5 (mm3/g). The experimental measurements have been considered as input of a detailed finite element model that couples moisture diffusion and hygroscopic expansion. The model allows to predict the response to the changes of relative humidity of a bi-layered composite made up of cellulose acetate and a non-hygroscopic textile
Influence of curing thermal history on cross-linking degree of a polydimethylsiloxane: Swelling and mechanical analyses
Full text linkIn this work, the change of the elastic properties induced by a change in cross-linking conditions of polydimethylsiloxane is investigated by measuring its shear modulus by dynamic mechanical analysis and correlating it to that predicted
from swelling measurements. Polymer cross-linking is performed at different curing temperatures reached with ramps at different heating rates. From both mechanical and swelling measurements, the molecular weight between cross-links, MC, is determined, and its dependency on the applied thermal history is analyzed. The main results are: (i) the elastic modulus of the cured material is not significantly affected by the heating rate adopted, while (ii) the curing temperature has a significant influence on the polydimethylsiloxane mechanical properties. In addition, (iii) MC evaluation from swelling measurements is in good agreement with that estimated from mechanical measurements when appropriate theories are considered. This last result suggests that swelling experiments can be considered as a reliable tool to predict the elastic modulus of the polydimethylsiloxane studied. The quantitative information reported in this paper, also obtainable by the suggested method if other thermal curing histories are applied, is extremely useful for the proper design of devices based on polydimethylsiloxane
Polydimethylsiloxane crosslinking kinetics: A systematic study on Sylgard184 comparing rheological and thermal approaches
Full text linkIn this work a systematic investigation of crosslinking kinetics of Sylgard184 polydimethylsiloxane is performed in both isothermal and dynamic conditions. The results are discussed in terms of two conversions, αC and αR determined by thermal and rheological analysis, respectively. Thermal analysis can well detect the first stage of the reaction, while rheological analysis starts being sensitive only at longer time. However, once the rheological response is observable, it changes with time faster than the calorimetric one. From rheology experiments it comes out that the gel point occurs at αR = 0.53, independently of the applied thermal history. At gel point, αC is around 0.30 indicating that about 30% of the bonds involved in the crosslinking process is enough to create an infinite network. A modified version of the Kamal's autocatalytic model allows to fit and predict the experimental findings from both the techniques; however, two distinct sets of parameters have been used. The results of this work may be a useful tool to design appropriate curing cycles for the preparation of Sylgard184
Moisture absorption measurement and modelling of a cellulose acetate
Full text linkWith a view toward the application of highly hygroscopic polymers as a humidity responsive self-actuator, the evaluation of the real time moisture concentration in the material becomes a priority. In this paper, the moisture diffusion process in a cellulose acetate (53.3% of acetylation) has been studied. Membranes of cellulose acetate (thickness within the range 66–200 μm) have been prepared, and the moisture absorption at room temperature and at a different relative humidity (RH within the range 21–53%) has been monitored. An analytical model has been used to describe the observed non-Fickian sigmoidal behavior of moisture diffusion. A relaxation factor (β) of about 0.026 s−1 and a moisture diffusion coefficient (D) of 3.35 × 10–6 mm2/s have been determined. At constant room temperature, the moisture concentration at saturation (Csat) has shown a linear relation with relative humidity. The identified values β, D and Csat of the analytical model have been used as input for the finite element simulation of the non-Fickian diffusion. The reliability of the finite element simulations has been confirmed with a second set of experiments
- …
