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Evaluation of mechanical properties of poly (lactic acid)/cellulose nanocrystal nanocomposites: A comparative study of conventional tensile test and small punch test
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Centre Català del Plàstic (CCP), Universitat Politècnica de Catalunya Barcelona Tech (EEBE-UPC), C/Colom, 114, Terrassa 08222, Spain.ORCID iD: 0000-0002-6857-4110
IUTA, Universidad de Oviedo, Edificio Departamental Oeste, N-7, 33203, Gijón, Spain.
Centre Català del Plàstic (CCP), Universitat Politècnica de Catalunya Barcelona Tech (EEBE-UPC), C/Colom, 114, Terrassa 08222, Spain.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0003-4762-2854
Show others and affiliations
2019 (English)In: Texto de las comunicaciones presentadas en el: Congreso del grupo español de fractura 2019, Grupo Español de fractura (GEF) , 2019, p. 226-231Conference paper, Published paper (Other academic)
Abstract [es]

El ácido poliláctico (PLA) ha ganado mucha atención debido a su potencial para sustituir algunos de los polímerosconvencionales basados en fuentes no renovables (petróleo), sin embargo, tiene algunos inconvenientes que restringen suuso. Una forma de mejorar las propiedades mecánicas y térmicas de PLA es mediante la adición de nanocargas como losnanocristales de celulosa (CNC).

La dispersión de los CNC en la matriz de PLA representa un desafío, ya que los CNC tienen una tendencia a aglomerarse.En este trabajo, se favoreció la dispersión de los CNC en la matriz de PLA mediante ultrasonidos y agitación, así comocon la modificación de la superficie de los CNC con la técnica de polimerización por apertura de anillo (ROP). Parafacilitar el procesamiento, se prepararon microesferas de PLA / CNC (1% en peso) mediante nitrógeno líquido.Finalmente, las películas de nanocompuestos se prepararon por extrusión-calandra.

El objetivo principal de esta investigación es analizar posibles efectos del refuerzo de los CNC sobre las propiedadesmecánicas del PLA. Las propiedades mecánicas de los nanocompuestos de PLA/CNC se evaluaron mediante ensayos detracción y análisis térmico mecánico dinámico (DMTA) y se compararon con los obtenidos utilizando el ensayo “smallpunch” (SPT).

Abstract [en]

Polylactic acid (PLA) has gained much attention due to its potential to substitute some of the oil-based polymers, however,it has some drawbacks which restrict its use. One way to improve the mechanical and thermal properties of PLA is byadding nanofillers such as cellulose nanocrystals (CNC).

The dispersion of the CNC in the PLA matrix is quite challenging because CNC has a tendency to agglomerate. In thiswork, CNC was well dispersed in the PLA matrix using ultrasonication and stirring. Furthermore, surface modificationof CNC was done by ring opening polymerization (ROP) technique to facilitate the dispersion. Then, the microspheresof PLA/CNC (1wt%) was prepared with liquid nitrogen for easy processing. Finally, nanocomposite films were preparedby extrusion calendering.

The main objective of this research is to see if there is any reinforcing effect of CNC on mechanical properties of PLA.Mechanical properties of PLA/CNC nanocomposites were evaluated by tensile testing and dynamic mechanical thermalanalysis (DMTA) and compared with the small punch test (SPT).

Place, publisher, year, edition, pages
Grupo Español de fractura (GEF) , 2019. p. 226-231
Series
Anales de Mecánica de la Fractura, ISSN 0213-3725 ; 36
Keywords [en]
Poly (lactic acid) (PLA), Cellulose nanocrystals (CNC), Nanocomposites, Small punch test (SPT)
National Category
Bio Materials
Research subject
Wood and Bionanocomposites
Identifiers
URN: urn:nbn:se:ltu:diva-73395OAI: oai:DiVA.org:ltu-73395DiVA, id: diva2:1301791
Conference
Congreso del Grupo Español de Fractura 2019 (GEF2019), 3-5 abril, 2019, Sevilla, España
Available from: 2019-04-02 Created: 2019-04-02 Last updated: 2023-09-05Bibliographically approved
In thesis
1. Properties of poly(lactic acid) in presence of cellulose and chitin nanocrystals
Open this publication in new window or tab >>Properties of poly(lactic acid) in presence of cellulose and chitin nanocrystals
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Plastic based materials are widely used for industrial and domestic packaging application. However, disposal of such petroleum-based materials e.g. polyethylene (PE), polypropylene (PP), and polyethyleneterephthalate (PET) has become a huge threat to the environment. These materials are non-biodegradable and complex for waste management, which causes plastic-pollution in both land and marine eco-system. For a sustainable industrial and economic development, it is indeed an urgency to develop packaging materials, which are environmentally benign, easy for waste treatment and recycling, and less/non-toxic. However, developing suitable and efficient plastic-substituents needs multiple requirements to be fulfilled viz., logistics and cost-effectiveness, good mechanical, thermal, optical and barrier properties, structural integrity of the constituents and morphological properties of the films. In this regard, utilizing bio-based polymers such as poly(lactic acid) (PLA), which originates from the natural resources, can be a viable and practical due to its low toxicity, biodegradability, and eco-friendly behaviour. Moreover, it has good optical and mechanical properties, e.g. high stiffness (3-4 GPa) and strength (50-70 MPa), which are similar or comparable to the polymers used for packaging applications. However, pristine PLA poses few challenges to overcome before it finds real world applications. Especially, slow crystallization rate, low crystallinity, poor toughness (very brittle material) and, poor barrier properties (O2 barrier) of PLA are particularly important aspects, which need to be modified and fine-tuned. Utilizing nano-reinforcements, such as nanocellulose and nanochitin, is a promising approach for modifying PLA because of raw materials abundancy; easily obtainable from forest-based and bio-waste, hence, utilizing such materials also help the sustainable bioeconomy. Chitin nanocrystals (ChNCs) and cellulose nanocrystals (CNCs) possess unique properties, such as, low density, biodegradability, low toxicity, good mechanical, and barrier properties; therefore, can act as suitable nano-reinforcements for PLA.

Homogeneous dispersion of the nano-reinforcements into the polymer matrix is crucial and challenging. To achieve good dispersion, primarily two methods were employed viz., (a) liquid–assisted extrusion of PLA with ChNCs in the presence of plasticizers, and (b) surface modification of the CNC via grafting. First segment of the research was aimed to understand and gain an insight about the role of nano-reinforcements on the crystallization behaviour of plasticized PLA e.g. crystallization kinetics including rate and temperature dependency, and morphology of the spherulites. ChNCs, due to large surface area, acted as better nucleating agent and improved the overall crystallization rate by reducing the crystallization time and size of the spherulites. Interestingly, rarely found neutral type of spherulites along with commonly occurring negative type, and multi ring-banded spherulites were observed at different crystallization time and temperature. Second part of the research was aimed to investigate the role of homogenously dispersed nano-reinforcements on the thermal, optical, barrier, and hydrolytic degradation properties of the nanocomposites. Noticeably, at a lower temperature (110 ºC), the highest rate of crystallization achieved within 5 min. Furthermore, homogenous crystallization and smaller spherulite size (7 nm) of PLA achieved due to the good dispersion of ChNCs significantly improved the crystallinity, thermal, barrier, and hydrolytic degradation properties. Faster crystallization at lower temperature resulted in a smaller spherulites sizes, which improved the oxygen and moisture barrier properties by hindering permeation path of the gases. On the other hand, the synergistic effect of isothermal crystallization and ChNCs improves the rate of hydrolytic degradation. It is noticeable that nanocomposites showed better optical properties than the plasticized PLA even at same crystallization conditions. As mechanical properties play an important role in packaging applications. So, the third part of the research involved the study of mechanical properties of oriented films (PLA/ChNCs) achieved by a combination of solid-state and melt-state drawings. Melt state drawing of relatively higher amount (5 wt%) ChNCs with PLA was prepared to obtain oriented films. These oriented nanocomposites films exhibited excellent mechanical properties. For example, a tensile strength with 360%, elongation at break with 2400%, and the toughness with 9500% increment achieved as compared to un-oriented nanocomposite films. The degree of crystallinity of highly oriented nanocomposite films increased from 8% to 53% with respect to the un-oriented nanocomposite films and smaller crystallites sizes were observed. Drawing conditions including drawing temperature and speed had a strong impact on the properties. By utilizing this knowledge, materials with high strength and toughness can be produced. Finally, in the fourth part, mechanical properties of the surface modified PLA/CNCs nanocomposites were investigated by a conventional tensile test and compared with the small punch test. Surface modification of CNC facilitated better dispersion of CNC into PLA matrix and increased the elastic modulus of the PLA/CNC nanocomposites. Grafting induced crazing effect, which induced better ductility. Knowledge and results gained in this study demonstrate the potential path for the development of the PLA nanocomposites with higher properties for packaging applications.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2020
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
poly(lactic acid), chitin nanocrystals, nanocellulose, crystallization, orientation, nanocomposites, mechanical properties-other properties
National Category
Bio Materials
Research subject
Wood and Bionanocomposites
Identifiers
urn:nbn:se:ltu:diva-79157 (URN)978-91-7790-615-5 (ISBN)978-91-7790-616-2 (ISBN)
Public defence
2020-10-09, E632, Luleå, 10:00 (English)
Opponent
Supervisors
Funder
EU, European Research Council
Available from: 2020-06-09 Created: 2020-06-03 Last updated: 2020-09-18Bibliographically approved

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Singh, ShikhaOksman, Kristiina

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