Petroula A. Tarantili

Physical properties of a silicone prosthetic elastomer stored in simulated skin secretions

STATEMENT OF PROBLEM Facial prostheses worn over an extended time are exposed to various environmental factors, including sebaceous oils (sebum) and perspiration. PURPOSE This study investigated the physical properties of tensile strength and modulus, elongation, tear strength, hardness, weight, and color change, of a silicone facial elastomer after immersion for 6 months in simulated sebum and perspiration at 37 degrees C. MATERIAL AND METHODS Specimens made of Episil silicone elastomer were immersed in simulated alkaline or acidic perspiration as well as in sebum. Tensile and tear tests were conducted according to ISO specifications no. 37 and 39, respectively, in a Monsanto testing machine. Shore A hardness measurements were run according to ASTM D 2240. Weight changes were followed at 5, 15, 30, and 180 days, and color changes were determined in the CIE LAB system using a tristimulus colorimeter. RESULTS An improvement of mechanical properties for specimens immersed in acidic perspiration was attributed to facilitation of the propagation of cross-linking reaction during aging of the silicone samples. Some weight increase was observed for the specimens immersed into the aqueous solution, whereas for those immersed in sebum, weight loss was recorded, probably because of extraction of some compounds. In this latter case, the color change was lower than that corresponding to simulated perspiration. CONCLUSION The silicone specimens aged for a period, which simulates 1.5 years of clinical service, showed minimal changes with respect to the properties studied.

Mechanical properties of epoxies reinforced with chloride-treated aramid fibers

Surface treatment of aramid fibers by immersion in a solution of methacryloyl chloride in carbon tetrachloride was carried out, and the resulting material was examined by means of electron microscopy and chemical analysis in an attempt to record any changes in the morphology and nature of the surface. Mechanical testing of tensile, flexural, and interlaminar shear strength, as well as dynamic mechanical analysis (DMA), were performed in an attempt to explore the effect of this treatment on the strength of the fiber. In a subsequent stage, the performance of those fibers as reinforcement in composites of epoxy matrix was assessed. The aim of this study was to provide more information about the interactions between the chloride-treated aramid fibers and the epoxy resin and, more specifically, to compare the behavior of the epoxy matrix composites with those composed of unsaturated polyester, polyethylene, and polyurethane matrix, which were studied in the past. It was found that specimens containing chloride-treated aramids display better flexural properties, whereas their tensile strength is drastically reduced. Improved performance was also identified by the DMA experiments.

Water sorption characteristics of epoxy resin-UHMPE fibers composites

Specimens of epoxy resin reinforced with ultrahigh-modulus polyethylene (UHMPE) fibers were immersed in water, and their swelling characteristics were recorded at various temperatures. In addition to an estimation of the response of those composite, the above study aimed at the exploration of the role of the fiber–matrix interface on the water sorption. Therefore, specimens containing original, calendered, and corona-treated fibers were tested. UHMPE fibers were found to limit the extent of sorption due to the nonhydrophilic character of polyethylene. However, specimens with poor interfacial properties, such as those with the original, untreated fibers, showed enhanced sorption since their surface area is drastically increased. As expected, the raise of temperature has a positive contribution to water sorption, and, furthermore, it seemed to affect the interface between epoxy and calendered fibers. On the other hand, the increase of filler volume fraction leads to a decrease in the amount of water uptake. The water transport in the neat epoxy resin specimens is rather diffusion-controlled, and this behavior was also recorded for the composite specimens reinforced with original UHMPE, which presented the maximum absorption.

Synthesis and Characterization of Low Molecular Weight Polylactic Acid

The polymerization of (D, L)-lactic acid in the absence of catalysts was studied. Azeotropic distillation using xylene, gave poly(lactic acid) with very low molecular weight, which however, was further increased by post-curing in an air oven. Moreover degradation phenomena and residual xylene were observed, with this procedure. Polycondensation of lactic acid at temperatures up to 180 C under vacuum, resulted in fast reaction with poor control and the products are still characterized by low molecular weight. When the reaction was run at 220 C in inert atmosphere, amorphous poly(lactic acid) was obtained with molecular weight exceeding 3000. The product was found pure, since it is free from solvents, catalysts and monomer. Also, no evidence of thermal degradation was observed. The above characteristics are well acceptable design aspects for biomedical uses.

The effect of accelerated ageing on performance properties of addition type silicone biomaterials.

The UV-protection provided to addition type silicone elastomers by various colorants, such as conventional dry earth pigments, as well as the so called “functional or reactive” pigments, was investigated. Moreover, the effect of a UV light absorber and a silica filler was also explored. Under the experimental parameters of this work, the exposure of silicone to UV radiation resulted in some changes of the IR absorbance, thermal decomposition after 400°C, Tg and tensile properties, whereas the storage modulus of samples was not affected. The obtained spectroscopic data, as well as the results of TGA and storage modulus, were interpreted by assuming that chain scission takes place during aging, whereas the improvement of tensile strength allows the hypothesis of a post-curing process, initiated by UV radiation. Therefore, the increase of Tg could partly be due to the above reason and, furthermore, to the contribution of a rearrangement of chain fragments within the free volume of the elastomeric material. Regarding the evaluation of various coloring agents used in this work, the obtained results show that dry pigments are more sensitive to accelerated ageing conditions in comparison with functional liquid pigments. Moreover, the hydrophobic character of silicone matrix is enhanced, with the addition of this type pigments because of the vinyl functional silanes groups present in their chemical structure. Finally, it should be noted that the incorporation of silica nanofiller did not seem to prevent the silicone elastomer from degradation upon UV irradiation, but showed a significant reinforcing effect.

Plasticized Poly(Vinyl Chloride) Filled with Waste Leather Particles

Waste leather granules were incorporated into poly(vinyl chloride) polymer plasticized with di-octyl phthalate. The blend was prepared by a melt mixing process using a banbury mixer. The rheological behaviour of the mixture was recorded and the filled samples were tested for their physicomechanical properties. It was found that density, wear resistance and Shore D hardness of the specimen increases, whereas the tensile properties are considerably deteriorated with the incorporation of leather particles. A concentration of 40 parts per hundred parts of resin was estimated as the upper level of leather content, satisfying the needs of the design of final products with potential applications to the footwear industry. In order to improve mechanical properties, some treatments of the leather granules were applied, such as sieving and coating with ethylene vinyl acetate copolymer solution. Both those treatments seem to improve tensile strength. An increase up to 30% was observed when coated granules were used.

Study of the off-axis properties of composites reinforced with ultra high modulus polyethylene fibres

Treated ultra-high-modulus polyethylene (UHMPE) fibres were used as reinforcement for epoxy resin and the obtained composites were evaluated. Corona and chromate surface-treated fibres, as well as calendered fibres were tested in terms of scanning electron microscopy (SEM), tensile and pull-out testing. Mechanical tests have also been performed with unidirectional specimens reinforced with the above fibres, focusing on those properties that are dependent on the off-axis properties of the composite, such as flexural and interlaminar shear strength (ILSS). Dynamic mechanical analysis (DMA) tests were also run. The results showed that corona and chromate treated fibres give the highest adhesive bonding, whereas their tensile strength is reduced. This is further observed with the flexural tests where maximum strength corresponds to these two types of fibres for various filler volume fractions. Finally, the data derived from ILLS and DMA suggest a slight advantage for the corona treated fibres.

In situ monitoring by DSC and modeling of curing of vinyl polysiloxanes in layered silicate nanocomposites

The study of curing mechanism and the related kinetics in montmorillonite/vinyl-terminated polysiloxane nanocomposites was carried out. Commercially unmodified montmorillonite, as well as two different types of organically modified products, under the trade names: Nanofil 116, Cloisite 20A and Cloisite 30B, respectively, was used as the reinforcing nanofillers. The vulcanization reaction was followed by isothermal and non-isothermal differential scanning calorimetry analysis. The Cloisite 20A/PDMS systems showed increased reaction rate at the early stages of cross-linking, followed by a retardation of the rate and extension of curing time, with respect to the pure PDMS. On the other hand, Cloisite 30B decreases the curing rate of PDMS during the whole process, whereas a significant increase in this parameter was recorded in the case of unmodified clay/PDMS nanocomposites. This diversity of the effect of various types of fillers was attributed to physicochemical interactions between the cross-linking system and organoclay particles mainly due to their surface chemistry. In an attempt to further evaluate the obtained results, it was found that the autocatalytic model shows good fitting with the experimental data.

Evaluation of thermal degradation mechanisms and their effect on the gross calorific value of ABS/PC/organoclay nanocomposites

In this work, nanocomposites of ABS/PC blends reinforced with organically modified montmorillonite (OMMT) were prepared by melt blending in a twin screw extruder, and an assessment of the thermal degradation mechanisms was performed by thermogravimetric analysis. The incorporation of PC improves the thermal resistance of ABS/PC blends, with respect to pure ABS. The addition of OMMT alters the degradation mechanism and modifies the properties of blends with higher PC content, where an increase of the degradation temperature corresponding to PC becomes obvious, in comparison with the respective unreinforced blends. The gross calorific value was calculated using an oxygen bomb calorimeter, and in most of the examined nanocomposites, an inverse trend was observed between this property and the residue calculated after thermogravimetric analysis in inert atmosphere. Based on the above results, the thermal degradation behavior of ABS/PC nanocomposites was interpreted by the heat barrier effect, caused via the formation of a carbonaceous silicate char, which insulates the underlying material creating a protective barrier to heat and mass transfer.

Reinforced Elastomers: Interphase Modification and Compatibilization in Rubber-Based Nanocomposites

An extended review is presented on the structure and properties of filler-matrix interface in reinforced elastomeric materials, since the above characteristics are critical for the overall performance of the related products. The current trends for interphase modification in rubber systems containing various fillers, such as Carbon black, Silica, Calcium Carbonate, Clays with emphasis on clay nanofillers, as well as Graphene is discussed. The use of fibrilar reinforcements is also reported, including traditional materials, such as Natural fibres, as well as Aramids and Carbon Nanotubes. On the other hand, the concept of hybrid composites, i.e., those composed of a mixture of matrices or reinforcements, further enhances the versatility of those materials, since it provides new possibilities of extending the area of rubber applications. In fact, the above products combine the property improvement attributed to each one of the system’s components and, moreover, they can usually take advantage of their synergistic action. In the same context the role of some other additives, necessary to adjust mechanical properties (e.g. plasticizers) or to promote phase miscibility in complex systems, such as compatibilizers, was investigated. The vulcanization of elastomeric materials is a critical step, with high impact on the properties of final products. In fact, the extent of this reaction, the cross-links density along with the other network parameters, are some important factors controlling the overall behavior of the vulcanized rubber and, therefore, monitoring, control, and modeling of rubber vulcanization are also examined in this work. The above review showed that the main reason for reinforcing rubbers is to improve their mechanical and thermal properties, as well as to reduce cost and sometimes the weight of a construction. It seemed that recent advances in nanoparticles have attracted much attention in manufacturing of rubber nanocomposites, because of the small size of filler and the corresponding increase in the surface area, which leads to the required mechanical properties at low filler loading. Carbon nanotubes and graphene nanoparticles are promising materials, offering good electrical properties. Surface treatment of the filler particles with the appropriate coupling agents is often vital, in order to promote proper dispersion and adequate filler/matrix interactions. Also, efficient dispersion of the reinforcement into rubber matrices usually needs the assistance of functionalized polymers, i.e., compatibilizers. Among the different modifying agents, maleic anhydride is the most commonly used and seems to ensure best results at relatively low cost. Finally, the cross-linking parameters must be controlled for an optimal network formation. The newly developed polyblends, based on mixtures of rubbers with polyolefins, require the suitable compatibilization in order to reveal their unique properties. Nanoparticles, being very efficient reinforcing agents even at low concentrations, were also found to play the role of compatibilizer for these mixtures of immiscible polymers.