Publication: Diseño, caracterización y comportamiento en servicio de materiales basados en ácido poliláctico (PLA) con potencial utilidad en el empaquetado de alimentos
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2016-02
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2016-02-03
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Abstract
En este trabajo se han preparado y caracterizado materiales nanocompuestos formados por la mezcla de nanorellenos de caolín o dióxido de titanio con una matriz polimérica termoplástica, Poli ácido láctico, PLA. El motivo por el cual se estudian estos materiales es su atractivo en relación a su potencialidad en el uso para aplicaciones agroalimentarias. Para ello, se prepararon materiales nanocompuestos termoplásticos dispersando nanopartículas mediante dos métodos: a) molienda de bolas de alta energía (HEBM) y, b) disolución y evaporación del disolvente “solvent-casting”. Finalmente, los materiales definitivos se prepararon en forma de películas mediante presión caliente. Este trabajo tiene como objetivo principal entender la influencia de la presencia de nanopartículas en las propiedades del PLA y las propiedades y prestaciones de los materiales nanocompuestos preparados. Para ello, entre otras cosas, los resultados obtenidos de la caracterización físico-química de los materiales se correlacionaron con sus propiedades termo-mecánicas finales así como su comportamiento en servicio frente al crecimiento de biofilms en su superficie. Los materiales utilizados en esta investigación son: Ácido Poliláctico (PLA), nanopartículas de Caolín (Al₄ Si₄O₁₀(OH)₈) y de Dióxido de titanio (TiO₂) para obtener nanocompuestos de PLA/Caolín y PLA/TiO₂ de diferente composición. Las composiciones estudiadas de los materiales compuestos de PLA/Caolín en tanto por ciento en peso fueron de 0% y 20% y para para el sistema PLA/TiO₂ de 0%, 1%, 5%, 10% y 20%, en este último caso con dos tamaños de partículas (∼ 21 y ∼ 100 nm), respectivamente. Mediante espectroscopia infrarroja por transformada de Fourier con reflectancia total atenuada (FTIR-ATR) y difracción de rayos X (XRD) se estudió la estructura de los materiales, mientras que por microscopía de fuerza atómica (AFM) y microscopía electrónica de barrido (SEM) se estudió la morfología de los materiales compuestos. Además, se estudiaron las propiedades mecánicas de los materiales PLA/TiO₂, analizándose el efecto de la presencia y porcentaje de TiO2 añadido. Las mejoras en las propiedades mecánicas del PLA se atribuyeron a un efecto de refuerzo de las nanopartículas, al menos para composiciones hasta el 5% en peso de nanopartículas pues se observó que para composiciones del 10% y 20% dichas propiedades eran inferiores. Además se observó una contribución favorable al disminuir tamaño o diámetro medio de las nanopartículas. En concreto, la presencia de TiO₂ a bajo contenidos y menor tamaño de partículas estudiado (∼ 21 nm de diámetro) incrementa la resistencia del material a la fractura. Una posible explicación sería un efecto reforzante debido a la transferencia de cargas de las partículas de TiO₂, favorecido por una mayor relación superficie-volumen de las nanopartículas y una dispersión uniforme de las mismas que depende a su vez de su buena distribución y orientación en el interior de la matriz polimérica. También se estudiaron transiciones térmicas durante los procesos de cristalización en frío (durante el calentamiento), fusión, cristalización (durante el enfriamiento) y termodegradación de los materiales mediante ensayos dinámicos utilizando calorimetría diferencial de barrido (DSC) y análisis termogravimétrico (TGA). Para estudiar el proceso de cristalización se consideró el segundo barrido de calentamiento y el contenido y tamaño de partículas. Para los dos sistemas preparados, PLA/Caolín y PLA/TiO₂ la temperatura de transición vítrea del PLA no presentó cambios significativos respecto al polímero puro sin partículas por lo que se pudo concluir que dichos rellenos no parecen influir en la movilidad o dinámica macromolecular del PLA, al menos en estado vítreo y a temperaturas cercanas a la de transición vítrea. Sin embargo, al estudiar los procesos de fusión y cristalización si se pudieron apreciar algunos cambios. Por ejemplo, las películas de PLA y PLA/Caolín molidas en condiciones criogénicas presentan mayor cristalinidad así como las películas de PLA/TiO₂. Además, un mayor tamaño de nanopartícula de TiO₂ parece contribuir favorablemente a la generación de cristales de PLA.
In this work nanocomposite materials based on a thermoplastic polymeric matrix, Polylactic Acid (PLA), filled with kaolin or titanium dioxide were prepared and characterized. The reason lies in their potential applications in agroalimentary industry. Nanocomposites were prepared by dispersing the nanoparticles in the polymer matrix using the following methods: a) high energy ball milling (HEBM), and b) solution and solvent evaporation "solvent-casting". Lastly, the final materials are obtained in the form of films by hot pressing. The aim of this work is to understand the influence of the presence of different nanoparticles in the final properties of PLA and PLA nanocomposites. The results obtained of the physico-chemical characterization of materials were correlated with their thermo-mechanical properties. The materials selected for this research were: Polylactic Acid (PLA), kaolin (Al₄ Si₄O₁₀(OH)₈) and titanium dioxide nanoparticles, TiO₂, to obtain binary systems based on the mixture of PLA/Kaolin and PLA/TiO2 with different composition. The composition selected for PLA/Kaolin systems was 20% (weight percent of kaolin particles) and for PLA/TiO₂ systems were 0%, 1%, 5%, 10% and 20% (weight percent of TiO₂ particles) using in this case two TiO₂ particle sizes (21 and < 100 nm). The structural analysis of the materials was carried out by means of Fourier transform infrared spectroscopy by attenuated total reflectance (FTIR-ATR) and X-ray diffraction (XRD) whereas atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to study the morphology of the composite materials. In addition, mechanical properties of the nanocomposites PLA/TiO₂ were studied analyzing the effects due to the presence of the nanoparticles as well as their content. The improved mechanical properties observed in PLA films were ascribed to a reinforcing effect due to the presence TiO₂ nanoparticles, at least for loads up to 5% (%wt/wt). However, for higher loads (10% and 20% of TiO₂, weight percentage) mechanical properties of PLA tend to be lower. These results suggest that low content of TiO₂ and small particle size (21 nm) increases the materials resistance to fracture favored by a higher surface to volume ratio of nanoparticles, i.e., an increase in the reinforcing effect through charge transfer TiO₂ particles is achieved. This result may be attributed to a uniform dispersion of the particles that depends on the good distribution and particle orientation in the polymer matrix. The thermal transitions of materials were determined by cold crystallization (heating), melting, crystallization (cooling) processes and thermal degradation from dynamic experiments run by differential scanning microscopy (DSC) and thermogravimetric analysis (TGA). To study the crystallization process the second heating scan was considered and the effects exerted by the presence of the nanoparticles (i.e., particle content) and their size were considered. For PLA/Kaolin and PLA/TiO₂ samples, the glass transition temperature did not show significant changes, suggesting that the presence of the particles does not seem to influence the dynamics and mobility macromolecular of PLA, at least in glassy state or at temperatures close to the glass transition. However, some changes were observed in the melting and crystallization processes. For example, PLA and PLA/Kaolin films milled under cryogenic conditions showed a higher crystallinity as well as PLA/TiO₂ films. This increase is greater in PLA/TiO₂ films with higher particle size (∼ 100 nm) and larger particle content. Additionally, studies on biofilm development on the surface of the films prepared were performed. For the PLA/Kaolin films it seems that the milling process is the main factor affecting the development of the biofilm, even more than the sole presence of kaolin particles. In general, irrespectively of the nanofillers used (kaolin or TiO₂), it seems that when there is a more ordered structure in the polymer matrix, there is a greater bacterial growth, i.e., changes in the development of biofilms on these materials appear to be due to changes in the surface properties induced by structural variations. Finally, a study at molecular scale of the effect exerted by the presence of TiO₂ nanoparticles as well as their particle size in the molecular dynamics of PLA was studied. These experiments were carried out by means of Fourier transform infrared spectroscopy in the near infrared region (FT-NIR) and using correlation spectroscopy in two dimensions (2D) to obtain additional information of the systems under study. Thermal transitions associated with the glass transition temperature (Tg ∼ 64 °C) and cold crystallization temperature (Tcc ∼ 123 °C) were clearly identified from analysis of integrated absorbance of the different bands. 2D correlation was performed based on the content of nanoparticles of TiO₂ (0, 1, 5 and 10%, weight percent) at three temperatures (30, 82 and 170 °C), respectively. The results suggest that the carbonyl group interacts specifically with TiO₂ nanoparticles. An increase of approximately 10 °C in the glass transition temperature for the samples with 1% of TiO₂ nanoparticles was observed. This result suggests a restriction on the movement of the chain of PLA due to the presence of TiO₂ nanoparticles, which could be caused by the specific interactions found between the carbonyl groups of the polymer and the nanoparticles. Similar results were observed when particle size of TiO₂ is analyzed.
In this work nanocomposite materials based on a thermoplastic polymeric matrix, Polylactic Acid (PLA), filled with kaolin or titanium dioxide were prepared and characterized. The reason lies in their potential applications in agroalimentary industry. Nanocomposites were prepared by dispersing the nanoparticles in the polymer matrix using the following methods: a) high energy ball milling (HEBM), and b) solution and solvent evaporation "solvent-casting". Lastly, the final materials are obtained in the form of films by hot pressing. The aim of this work is to understand the influence of the presence of different nanoparticles in the final properties of PLA and PLA nanocomposites. The results obtained of the physico-chemical characterization of materials were correlated with their thermo-mechanical properties. The materials selected for this research were: Polylactic Acid (PLA), kaolin (Al₄ Si₄O₁₀(OH)₈) and titanium dioxide nanoparticles, TiO₂, to obtain binary systems based on the mixture of PLA/Kaolin and PLA/TiO2 with different composition. The composition selected for PLA/Kaolin systems was 20% (weight percent of kaolin particles) and for PLA/TiO₂ systems were 0%, 1%, 5%, 10% and 20% (weight percent of TiO₂ particles) using in this case two TiO₂ particle sizes (21 and < 100 nm). The structural analysis of the materials was carried out by means of Fourier transform infrared spectroscopy by attenuated total reflectance (FTIR-ATR) and X-ray diffraction (XRD) whereas atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to study the morphology of the composite materials. In addition, mechanical properties of the nanocomposites PLA/TiO₂ were studied analyzing the effects due to the presence of the nanoparticles as well as their content. The improved mechanical properties observed in PLA films were ascribed to a reinforcing effect due to the presence TiO₂ nanoparticles, at least for loads up to 5% (%wt/wt). However, for higher loads (10% and 20% of TiO₂, weight percentage) mechanical properties of PLA tend to be lower. These results suggest that low content of TiO₂ and small particle size (21 nm) increases the materials resistance to fracture favored by a higher surface to volume ratio of nanoparticles, i.e., an increase in the reinforcing effect through charge transfer TiO₂ particles is achieved. This result may be attributed to a uniform dispersion of the particles that depends on the good distribution and particle orientation in the polymer matrix. The thermal transitions of materials were determined by cold crystallization (heating), melting, crystallization (cooling) processes and thermal degradation from dynamic experiments run by differential scanning microscopy (DSC) and thermogravimetric analysis (TGA). To study the crystallization process the second heating scan was considered and the effects exerted by the presence of the nanoparticles (i.e., particle content) and their size were considered. For PLA/Kaolin and PLA/TiO₂ samples, the glass transition temperature did not show significant changes, suggesting that the presence of the particles does not seem to influence the dynamics and mobility macromolecular of PLA, at least in glassy state or at temperatures close to the glass transition. However, some changes were observed in the melting and crystallization processes. For example, PLA and PLA/Kaolin films milled under cryogenic conditions showed a higher crystallinity as well as PLA/TiO₂ films. This increase is greater in PLA/TiO₂ films with higher particle size (∼ 100 nm) and larger particle content. Additionally, studies on biofilm development on the surface of the films prepared were performed. For the PLA/Kaolin films it seems that the milling process is the main factor affecting the development of the biofilm, even more than the sole presence of kaolin particles. In general, irrespectively of the nanofillers used (kaolin or TiO₂), it seems that when there is a more ordered structure in the polymer matrix, there is a greater bacterial growth, i.e., changes in the development of biofilms on these materials appear to be due to changes in the surface properties induced by structural variations. Finally, a study at molecular scale of the effect exerted by the presence of TiO₂ nanoparticles as well as their particle size in the molecular dynamics of PLA was studied. These experiments were carried out by means of Fourier transform infrared spectroscopy in the near infrared region (FT-NIR) and using correlation spectroscopy in two dimensions (2D) to obtain additional information of the systems under study. Thermal transitions associated with the glass transition temperature (Tg ∼ 64 °C) and cold crystallization temperature (Tcc ∼ 123 °C) were clearly identified from analysis of integrated absorbance of the different bands. 2D correlation was performed based on the content of nanoparticles of TiO₂ (0, 1, 5 and 10%, weight percent) at three temperatures (30, 82 and 170 °C), respectively. The results suggest that the carbonyl group interacts specifically with TiO₂ nanoparticles. An increase of approximately 10 °C in the glass transition temperature for the samples with 1% of TiO₂ nanoparticles was observed. This result suggests a restriction on the movement of the chain of PLA due to the presence of TiO₂ nanoparticles, which could be caused by the specific interactions found between the carbonyl groups of the polymer and the nanoparticles. Similar results were observed when particle size of TiO₂ is analyzed.