Fernando Carrillo

Crystallinity changes in lyocell and viscose-type fibres by caustic treatment

One of the most important treatments performed on cellulosic fibres to improve properties such as dimensional stability, tensile strength and lustre, is mercerisation. The aim of this work was to study the crystallinity, accessibility and unit cell structure changes occurring in three types of regenerated cellulose fibres (lyocell, modal and viscose) that were mercerised with caustic soda solutions of different concentrations. Differences were observed between the behaviour of the viscose type fibres (viscose and modal) and that of the lyocell fibres. For the viscose type fibres, the proportion of crystalline regions increased at low alkali concentrations, while for lyocell fibres a decrease in crystallinity was observed. In all three fibres there was a transformation from cellulose II to amorphous cellulose. While for lyocell the transformation was partial, the modal and in particular the viscose fibres showed a complete transformation, and the swelling agent caused the fibre to dissolve at high caustic concentrations.

Nanoindentation of polydimethylsiloxane elastomers: Effect of crosslinking, work of adhesion, and fluid environment on elastic modulus

With the potential to map mechanical properties of heterogeneous materials on a micrometer scale, there is growing interest in nanoindentation as a materials characterization technique. However, nanoindentation has been developed primarily for characterization of hard, elasto-plastic materials, and the technique has not been validated for very soft materials with moduli less than 5 MPa. The current study attempted to use nanoindentation to characterize the elastic moduli of soft, elastomeric polydimethylsiloxane (PDMS) samples (with different degrees of crosslinking) and determine the effects of adhesion on these measurements using adhesion contact mechanics models. Results indicate that nanoindentation was able to differentiate between elastic moduli on the order of hundreds of kilo-Pascals. Moreover, calculations using the classical Hertz contact model for dry and aqueous environment gave higher elastic modulus values when compared to those obtained from unconfined compression testing. These data seem to suggest that consideration of the adhesion energy at the tip-sample interface is a significantly important parameter and needs to be taken into account for consistent elastic modulus determination of soft materials by nanoindentation.

Simulated Soft Tissue Nanoindentation: A Finite Element Study

To address the growing interest in nanoindentation for biomaterials, the following finite element study investigated the influence of indentation testing protocol and substrate geometry on quasi-static and dynamic load-displacement behavior of linear viscoelastic materials. For a standard linear solid, the conventional quasi-static indentation modulus, E QS , fell between the instantaneous and equilibrium modulus of the model. E QS approached the equilibrium modulus only for indentation unloading times 1000 times greater than the characteristic relaxation time of the model. It was nearly insensitive to other changes in the indentation testing protocol, such as tip radius and penetration depth, exhibiting variations of only 5–10%. Dynamic nanoindentation provided a quantitatively accurate assessment of the complex dynamic modulus (within ±12%) for a range material of parameters at physiologically relevant testing parameters. Both quasi-static and dynamic moduli calculated from the irregular surfaces varied with the size and shape of the irregularities but were still within 10% of the smooth surface values for penetration depths larger than the dimensions of the surface irregularities.

Resolving the Electrospinnability Zones and Diameter Prediction for the Electrospinning of the Gelatin/Water/Acetic Acid System

The development of suitable biomimetic scaffolds is a fundamental requirement of tissue engineering. Although electrospinning has emerged as an effective method for producing such scaffolds of nanometer-sized fibers, the influence of solution characteristics on the morphology of the resulting nanofibers depends on each polymer solution system. In this study, gelatin nanofibers and microfibers were prepared via electrospinning using mixtures of water and acetic acid at different ratios as solvents. The viscosities of gelatin solutions before electrospinning were analyzed and two different behaviors were found as a function of the solvent composition, taking into account classic models of polymer science. A power law relationship between viscosity and gelatin concentration was found for each solvent system, and an empirical model including the influence of acetic acid was obtained for aqueous systems. Moreover, a ternary diagram considering gelatin, water, and acetic acid mass fractions was constructed as a tool to establish the electrospinnability domains in terms of fiber occurrence and morphology. Also, the isodiametric curves were defined in the fibers region. Finally, in order to correlate the diameter of electrospun nanofibers and the electrospinnability zones, the Berry number was used. However, as its only allows the range of electrospinnability to be established for a fixed solvent composition, a new dimensionless parameter (Bemod) was suggested to take into account all the acetic acid aqueous solutions as a single solvent.

The Use of Waxes and Wetting Additives to Improve Compatibility Between HDPE and Ground Tyre Rubber

The concern about the amounts of used tyres that end in landfills and the environmental problems that they produce have led to research on ways to use these residues in common applications. A general starting point in the recovering procedures is the grinding of the tyres in order to obtain a powder known as ground tyre rubber (GTR). This paper proposes an alternative way of recycling the used tyres by means of a new material composed by a polyolefin matrix, high-density polyethylene (HDPE) reinforced with GTR. The crosslinked structure and surface characteristics of the GTR hampers good interaction with the polymeric matrix, leading to weak adhesion between the two phases and consequently a decrease of the mechanical properties. The aim of this work is to improve adhesion, achieving better wetting of GTR particles by HDPE. For this purpose, waxes and wetting additives were added to the mixture. The influence of these additives was determined by mechanical properties, attenuated total reflection infrared spectroscopy, differential scanning calorimetry analysis, scanning electron microscopy, and optical microscopy. An acidic wetting additive and a polyethylene wax showed good results regarding mechanical properties, dispersion of the GTR particles and adhesion.

Acoustic and mechanical properties of recycled polyvinyl chloride/ground tyre rubber composites

In order to provide another way of reducing the stock of used tyres and polyvinyl chloride waste, a new material is developed and studied. Formulation includes a matrix constituted by a compound of recycled polyvinyl chloride with plasticized polyvinyl chloride and a reinforcement of ground tyre rubber. Acoustic and mechanical properties of different compositions of polyvinyl chloride/ground tyre rubber were tested in order to determine their suitability for applications fulfilling industry requirements. Sound absorption has been analyzed, showing interesting results at frequencies higher than 2500 Hz. The obtained values are found to be depending on the thickness of the sample, the content of the ground tyre rubber and the existence of gaps, pores and voids either between layers or in the interphase between the matrix and reinforcement. From the study of the mechanical properties, we may observe that the ground tyre rubber act as filler, improving stiffness of polyvinyl chloride/ground tyre rubber composites with an increase of Young Modulus. The tensile strength, elongation at break and toughness decrease slowly. The decrease of these mechanical properties is observed to be lower than in the case of composites made by using high-density polyethylene as a matrix.

Biocomposites using waste whole chicken feathers and thermoplastic matrices

This study deals with the preparation and characterization of thermoplastic composites using polypropylene, high-density polyethylene and polylactic acid matrices and including whole chicken feathers as reinforcement. The behaviour of the composites was determined in terms of physical and mechanical properties, which were related to the fibre–matrix compatibility analysed by Fourier transform infrared spectroscopy and scanning electron microscopy. The results showed that the addition of chicken feathers into the thermoplastic matrices results in a slight increase in the stiffness when small amounts of chicken feathers (5–10% vol/vol) were incorporated into the composites. Tensile strength at maximum load, elongation at break and toughness properties decreased when the chicken feather concentration was increased. Results for chicken feather–polypropylene composites were analogous to chicken feather–high-density polyethylene and chicken feather–polylactic acid composites. The Fourier transform infrared spectroscopic study and the scanning electron micrographs suggest that the insufficient compatibility of chicken feather and polymer matrices is the main reason for the decrease in tensile properties.

Properties and optimal manufacturing conditions of chicken feathers/poly(lactic acid) biocomposites

Chicken feathers waste from poultry industry was incorporated in poly(lactic acid) matrix to obtain an environmental friendly biocomposite taking advantage of the unique properties of chicken feathers, such as low density, biodegradability and good thermal and acoustic properties, and of the biodegradability of the poly(lactic acid). The effect of manufacturing conditions on the final properties of the composite and on the matrix–fiber compatibility was studied. Optimal manufacturing conditions, in order to obtain the best mechanical results, were found at a temperature of 170–180℃ for a processing time of 5 min and a speed of mixing of 50 r/min. Young’s modulus was not very affected by the chicken feather’s content showing a maximal variation of less than 8%, indicating that is possible to include chicken feathers in a composite maintaining its stiffness. However, tensile strength and elongation decreased up to 58 and 12%, respectively, when chicken feather content was 25% because of the restraining effect of the fibers. Moreover, dimensional stability was negatively affected with the inclusion of chicken feathers. Infrared spectroscopy and scanning electron microscopy studies show that fiber–matrix interaction exists but it is weak.

High modulus regenerated cellulose fiber-reinforced cellulose acetate butyrate biocomposites

The properties of composites prepared with a matrix of biodegradable cellulose ester (cellulose acetate butyrate, CAB) and reinforced with regenerated cellulose lyocell fibers (lyocell/CAB) were studied and compared with short flax fiber-reinforced composites (flax/CAB), used as reference. The effect of the lyocell fiber content on the composite properties was also investigated. Tensile properties, dimensional stability, fiber—matrix compatibility, and biodegradability were investigated by tensile testing, water absorption test, scanning electron microscopic analysis, and soil burial test, respectively. From the results, it was shown how the Young’s modulus of lyocell/CAB composites increased from 2 GPa for neat CAB to 4 GPa for a composite with a lyocell fiber content of 34.8% (v/v). Similar trend was obtained for flax/CAB biocomposites which showed higher modulus than lyocell/CAB composites, with values of 5 GPa for a flax/CAB composite with the same composition. Moreover, tensile strength of lyocell/CAB composites with fiber content higher than 16.7% (v/v) resulted in lower values than neat CAB, indicating a high probability of failure cracks on lyocell/CAB composite samples when increasing fiber composition. In addition, compared to neat CAB, elongation at break decreased for all the composites studied. It was also observed that increasing the fiber content, the water absorption of the composites increased compared to neat CAB matrix due to the hydrophilic nature of the lyocell and flax fibers. The biodegradation test showed, after 60 days of soil burial, about 10% and 25% of mass lost for 34.8% (v/v) of lyocell/ CAB and flax/CAB composites, respectively.

Properties and optimal manufacturing conditions of chicken feathers thermoplastic biocomposites

The aim of this study was the analysis and characterization of composites based on thermoplastics (ethylene vinyl acetate, polypropilene and high-density polyethylene) and chicken feathers. Several composite samples with a content of 20% v/v of chicken feathers have been studied to determine the optimal manufacturing conditions of temperature, mixing time, and mixing speed to achieve the best tensile properties. The results have shown that the addition of micronized chicken feather (20% v/v) to thermoplastic matrices increases stiffness and provides a more brittle behavior. Ethylene vinyl acetate matrix also shows an ability to participate in second-order intermolecular interactions with chicken feathers, providing better tensile properties (tensile strength and toughness) than polypropilene and high-density polyethylene. Optimal manufacturing conditions were found for a mixing time of around 5 min; a mixing speed of 50 r min−1; and temperature values of 160℃ in case of high-density polyethylene, 120℃ for ethylene vinyl acetate, and 170℃ for polypropilene. Fourier transform infrared spectroscopy, differential scanning calorimetry and scanning electron microscopy analysis have been performed in order to provide further understanding of the compatibility and microstructural features that support the tensile properties of the materials.