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You are here: Home > Technique > Processes > Scientific report of the LGP2 > Packaging and converting > Cellulose and starch reinforced composites and nanocomposites           Update: April 26th 2007
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Researchers of the LGP2 (EFPG, INPG, CNRS, CTP)
(November 2006)
 
Documents taken from the
"Scientific Report of the Laboratory of Pulp and Paper Science and Graphic Arts - UMR 5518
Grenoble - France
January 2002-November 2005"

V - Packaging and converting

V - 3 - Cellulose and starch reinforced composites and nanocomposites
Wim Thielemans, Ly Babacar, Alain Dufresne, Naceur Belgacem

The development of composite materials from renewable and biodegradable materials has for several years been receiving more and more attention fed by the current drive towards global sustainability and the rising cost of crude oil. Cellulose and starch, as some of the most widely available renewable materials, have been the focus of a large fraction of work in this effort. The excellent mechanical composure of cellulose makes that it has largely been investigated as a reinforcing fibre, while starch has largely been studied as matrix material in a gelatinised, plasticized state. Despite the excellent potential of these materials, important shortcomings are found in their moisture sensitivity and incompatibility with oleophilic polymers. Additional important advantages are found in their relatively low density, low cost and worldwide availability, albeit in different form. Reinforcement of polymeric materials with rigid organic and inorganic particles displaying length scales below the micrometer level are currently receiving enormous attention from both the academic and the industrial world.
The wide interest in the obtained materials, commonly referred to as “nanocomposites”, can be found in a number of important effect related to the small size of the reinforcing particles:

Despite the major advantages of nanoparticles, their use is still restricted to a niche of specialized applications. The main causes can be found in the limited availability of the nanoparticles, but also in their tendency to aggregate which hinders the homogeneous dispersion in the polymer matrix needed to achieve the behaviour describe before.

The research being performed in our research group focuses on solving these problems in two stages. The first is the acid hydrolysis of cellulosic materials, chitin and starch to various extents up to the complete destruction of the amorphous phase, resulting in nanosized monocrystals., while the second stage focuses on their chemical modification. The morphology and aspect ratio of polysaccharide nanocrystals depend on the source material and its origin. An important effort of our work is directed towards deriving and characterising nanocrystals from novel polysaccharide source materials.

Novel potential applications for nanocomposites are also being investigated. An example can be found in a joint PhD project with LEPMI (Laboratoire d’Electrochimie et de Physico-chimie des Matériaux et des Interfaces), where cellulose nanoparticles were considered as reinforcement for electrolytic nanocomposites to be used in lithium batteries. The electrolyte thickness could be reduced by a factor of 100 without compromising the conductivity or safety. The expected cost and internal resistance savings of this thickness reduction are considerable.

Starch nanoparticles have been investigated in the framework of another PhD project. The preparation of these new nanoparticles was optimized, after which they were used as reinforcement in natural rubber and thermoplastic starch. It was found that the reinforcing effect of starch nanocrystals is significantly less than is the case for their cellulose and chitin counterparts, attributed to a difference in particle shape (starch nanoparticles are obtained as platelets, while cellulose and chitin nanoparticles are rigid rod shapes). Nevertheless, the worldwide abundance of starch, the simplicity of the hydrolysis process, and the absence of a direct influence of nanocomposites fabrication on final macroscopic properties, (this latter point is to be further investigated) provide ample reason to expect the development of a vast array of applications based on these materials.

The shape, size and aspect ratio of these nanocrystals depends on the source material: cellulose nanocrystals are rigid rods with average diameters and lengths are found to vary between 3-20 nm and 100-2000 nm respectively, while starch nanocrystals are recovered as platelets of 5 nm thickness and widths between 20-50nm.

The second stage investigates various methods to chemically modify these partially or completely hydrolysed substrates. These modifications are préexpected to improve the fibre-matrix interface. Additionally, the modification should also provide an efficient hydrophobic barrier to avoid moisture uptake and the consequent loss of mechanical properties. The chemical functionalities to be employed are targeted towards reaction with the cellulose and starch surface hydroxyl groups. They include siloxanes, isocyanates, carboxylic anhydrides and epoxides. Non-swelling reaction media are being used in order to guarantee the modification reaction to be restricted to the surface.

Characterisation techniques to confirm chemical reaction include FTIR and XPS spectroscopy, contact angle measurements, and elemental analysis. X-ray diffraction is being used to confirm the retention of the crystalline structure of the reinforcement particles after reaction, while differential scanning calorimetry experiments clarify whether the modification of partially hydrolysed particles was successful in plasticising the modified structure.

The following procedures are employed:

[Figures 1-6] show some results taken as examples and illustrating the most relevant advances achieved in the present context.

Transmission electron micrographs of cellulose. Cellulose nanocrystals were derived from sisal   Transmission electron micrographs of starch nanocrystals
Figure 1 - Transmission electron micrographs
of cellulose. Cellulose nanocrystals
were derived from sisal
  Figure 2 - Transmission electron
micrographs of starch nanocrystals
Size distribution of sisal nanocrystals (length)   Size distribution of sisal nanocrystals (diameter)
Figure 3 - Size distribution
of sisal nanocrystals (length)
  Figure 4 - Size distribution of
sisal nanocrystals (diameter)
Glass transition temperature of polyvinyl acetate/sisal whisker composites
Figure 5 - Glass transition temperature of polyvinyl acetate/sisal
whisker composites as a function of relative humidity
0 (_), 35 (∅), 43 (Õ), 58 (_), 75 (_), and 98%RH (+))
and whisker content (0 (_), 1 (◊), 2.5 (∅), 5 (Õ), and 10 wt% (_))
Water contact angle measurements of starch nanoparticles
Figure 6 - Water contact angle measurements of starch nanoparticles:
(a) unmodified, and modified with (b) stearoyl acid chloride and (c) polyethylene glycol

Nanocomposites are subsequently formed with the hydrolysed materials, with or without chemical modification. Three different approaches are under investigation:

 
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