V. Lorenzo

Abrasion resistance in the Tumble test of sol–gel hybrid coatings for ophthalmic plastic lenses

Hard abrasion-resistant coatings for ophthalmic plastics were obtained from mixtures of tetraethoxysilane, 3-metacryloxypropyltrimethoxysilane and methyl methacrylate, and were cured at 120°C and 140°C for different times. The Tumble test was used to measure the abrasion resistance of these coated plastics. This abrasion resistance was strongly dependent on the curing time and temperature. The infrared analysis of the curing processes showed that, in order to achieve the highest abrasion resistance, these acrylic hybrid coatings must be fully polymerized, but overcuring must be carefully avoided. Finally, a linear correlation was found between the abrasion resistance of these coated samples (as measured by the Tumble test) and their microhardness.

Influence of annealing on the thermal and viscoelastic behaviour of poly(triethylene glycol p,p′-dibenzoate)

Abstract The effect of thermal history on the phase behaviour and viscoelastic relaxations of poly(triethylene glycol p , p ′-dibenzoate) (PTEB) was studied. The mesophase of PTEB can be converted into a three-dimensional crystal by annealing at appropriate temperatures and for appropriate times. Three PTEB samples were analysed, one of them freshly quenched from the melt and the other two annealed under different conditions (room temperature and 85°C). The phase structure was investigated by differential scanning calorimetry and X-ray diffraction, both at wide and low angles (using synchrotron radiation in the latter case). This structure affects slightly the β and γ relaxations, whereas the glass (α) transition is more intense and narrow and appears at a lower temperature for the sample freshly quenched from the melt, which exhibits only the liquid crystalline phase.

Study of the effects of the reaction conditions on the modification of clays with polyelectrolytes and silanes

New organically modified clays have been obtained from sodium montmorillonite, using either a cationic polyelectrolyte (polyethylenimine) or a novel homemade bisphenol-A silane as modifiers. The modification processes have been carried out in different reaction media, in order to study the effects on the properties of the modified clays of several reaction parameters, such as the pH of the polyethylenimine solution or the nature of the solvent used in the silanization. The obtained clays were characterized using X-ray diffraction, thermogravimetric analysis, and FTIR spectroscopy. Clays modified with polyelectrolyte or silane show significant increases in the basal spacing. The properties of polyelectrolyte-modified clays depend on the pH of the treating solution. The increase in the basal spacing of polyelectrolyte-modified clays varies only slightly with the pH; however, this reaction parameter clearly determines the total amount of polyelectrolyte introduced in the clay. The properties and applications of silane-modified clays are strongly dependent on the presence of water in the reaction media used for the silanization. These results have been explained by considering that the reaction conditions determine the nature and the amount of material intercalated into the clay.

Delayed elastic recovery of hardness indentations in polyethylene

The depth of Vickers microindentations in high density, low density and linear low density polyethylenes has been studied by two wave interferometry. The evolution of the delayed depth recovery is explained in terms of the Burgers model of viscoelastic behaviour and the numerical parameters associated with the model are related to the crystallinity, elastic modulus and yield stress of the polyethylenes studied.

Analysis of the activation energy spectrum for the enthalpy relaxation of a glassy liquid crystalline polymer

The phenomenology of the physical ageing of glassy materials can be described by using the activation energy spectrum model. This model was originally developed for relaxations in metallic and oxide glasses and this paper applies it to a liquid crystalline polymer, poly(diethylene glycol p, p′-bibenzoate) (PDEB), in the glassy state.PDEB samples were quenched from above Tg and isothermally annealed in a differential scanning calorimetry (DSC) equipment, and a description of the enthalpy relaxation of PDEB was achieved by means of the Kohlrausch–Williams–Watts (KWW) equation. The results show that the relaxation time decreases when the annealing temperature increases and can be explained in terms of an Arrhenius-type equation. The predicted value for the apparent activation enthalpy (0.98 eV) is lower than those reported for other polymers. This approach assumes that a unique elemental process with a relaxation time τ0=1.3×10-13 min controls the evolution of the system.Energy spectra demonstrate that the activation energy values for the mechanisms controlling the relaxation are lower than 1.1 eV and the spectra present a maximum for an activation energy close to 0.98 eV. The total number of available relaxation processes (measured by the area below the curves) decreases if the sub-Tg annealing temperature increases. Moreover, the results show that the distributions widen if the shape parameter of the KWW equation decreases.

Microhardness of polyethylene surface modified by chlorosulphonic acid

Chlorosulphonic acid etching of polyethylene produces changes in its surface structure that can be tested by means of microhardness measurements. The results of different microhardness parameters (Mayers lines, penetration curves, Vickers microhardness and total microhardness) are compared, discussed and related to the structure of polyethylene.

Assessment of mechanical behavior of PLA composites reinforced with Mg micro-particles through depth-sensing indentations analysis.

This work deals with the mechanical characterization by depth-sensing indentation (DSI) of PLLA and PLDA composites reinforced with micro-particles of Mg (up to 15wt%), which is a challenging task since the indented volume must provide information of the bulk composite, i.e. contain enough reinforcement particles. The composites were fabricated by combining hot extrusion and compression moulding. Physico-chemical characterization by TGA and DSC indicates that Mg anticipates the thermal degradation of the polymers but does not compromise their stability during processing. Especial emphasis is devoted to determine the effect of strain rate and Mg content on mechanical behavior, thus important information about the visco-elastic behavior and time-dependent response of the composites is obtained. Relevant for the intended application is that Mg addition increases the elastic modulus and hardness of the polymeric matrices and induces a higher resistance to flow. The elastic modulus obtained by DSI experiments shows good agreement with that obtained by uniaxial compression tests. The results indicate that DSI experiments are a reliable method to calculate the modulus of polymeric composites reinforced with micro-particles. Taking into consideration the mechanical properties results, PLA/Mg composite could be used as substitute for biodegradable monolithic polymeric implants already in the market for orthopedics (freeform meshes, mini plates, screws, pins, …), craniomaxillofacial, or spine.

Anomalous orientation of poly(trimethylene oxy-2 methyl trimethylene p–p′ bibenzoate) as revealed by means of tensile tests

Abstract Poly(trimethylene oxy-2 methyl trimethylene p – p ′ bibenzoate) is a linear thermotropic liquid crystal polymer that may or may not develop anomalous orientation depending on the applied strain rate. X-ray diffraction patterns show that macromolecular chains tend to align perpendicularly to the deformation direction for the slowest analysed strain rates and in parallel to the deformation direction for the highest rates. The study of the strain–stress curves reveals that plastic deformation of this polymer can be controlled by two different microscopic mechanisms whose relative contributions to the overall strain are rate dependent.

Effect of simulated mechanical recycling processes on the structure and properties of poly(lactic acid)

The aim of this work is to study the effects of different simulated mechanical recycling processes on the structure and properties of PLA. A commercial grade of PLA was melt compounded and compression molded, then subjected to two different recycling processes. The first recycling process consisted of an accelerated ageing and a second melt processing step, while the other recycling process included an accelerated ageing, a demanding washing process and a second melt processing step. The intrinsic viscosity measurements indicate that both recycling processes produce a degradation in PLA, which is more pronounced in the sample subjected to the washing process. DSC results suggest an increase in the mobility of the polymer chains in the recycled materials; however the degree of crystallinity of PLA seems unchanged. The optical, mechanical and gas barrier properties of PLA do not seem to be largely affected by the degradation suffered during the different recycling processes. These results suggest that, despite the degradation of PLA, the impact of the different simulated mechanical recycling processes on the final properties is limited. Thus, the potential use of recycled PLA in packaging applications is not jeopardized.

Effects of Aging and Different Mechanical Recycling Processes on the Structure and Properties of Poly(lactic acid)-clay Nanocomposites

The growing use of poly(lactic acid) (PLA) and PLA-based nanocomposites in packaging has raised the interest of studying the mechanical recycling of the wastes and the properties of the recycled materials. The main objective of this work was to study the effect of two different mechanical recycling processes on the structure and properties of a PLA-montmorillonite nanocomposite. The two recycling processes included accelerated thermal and photochemical aging steps to simulate the degradation experienced by post-consumer plastics during their service life. One of them also included a demanding washing process prior to the reprocessing. A decrease in the molecular weight of PLA was observed in the recycled materials, especially in those subjected to the washing step, which explained the small decrease in microhardness and the increased water uptake at long immersion times. Water absorption at short immersion times was similar in virgin and recycled materials and was accurately described using a Fickian model. The recycled materials showed increased thermal, optical and gas barrier properties due to the improved clay dispersion that was observed by XRD and TEM analysis. The results suggest that recycled PLA-clay nanocomposites can be used in demanding applications.