Properties of poly(lactic acid) in presence of cellulose and chitin nanocrystals [Elektronisk resurs]

• 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. 

Ämnesord

Engineering and Technology  (hsv)

Industrial Biotechnology  (hsv)

Bio Materials  (hsv)

Teknik och teknologier  (hsv)

Industriell bioteknik  (hsv)

Biomaterial  (hsv)

Trä och bionanokompositer  (ltu)

Wood and Bionanocomposites  (ltu)

Genre

government publication  (marcgt)

Indexterm och SAB-rubrik

poly(lactic acid)

chitin nanocrystals

nanocellulose

crystallization

orientation

nanocomposites

mechanical properties-other properties