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Researchers of the LGP2
(May 2011)
 
Grenoble INP-Pagora, International school of paper, print media and biomaterials
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Laboratory of Pulp and Paper Science and Graphic Arts

V - Converting Biomaterials Packaging

V - 2 - Bionanocomposites: from new sources to new processes

The field of nanotechnologies is growing up very fast. The hierarchical structure of cellulose allows considering the extraction of nanoparticles having different morphologies. We work towards the maintain of our advance in this domain since our implication in this field started in 1993 and we act as pioneers. We also continue our efforts to identify new sources of polysaccharides able to lead to nanoparticles with interesting properties. During the last years we have in particular varied the processing techniques of nanocomposite materials reinforced with these nanoparticles. For instance, we have tried to graft ("grafting onto" approach) or to grow polymeric chains at the surface of the nanoparticles ("grafting from" approach).

Introduction

The processing of polymeric materials filled with polysaccharide nanoparticles, in particular based on cellulose, but also starch and chitin, mobilize scientific communities. "Cellulose nanocomposites" or "nanocellulose" are becoming topical. Cellulose nanoparticles are extracted from cellulosic fibers such as those used for the production of paper and occur originally in the form of stable colloidal suspensions of cellulosic fragments. These fragments occur as rod-like nanoparticles in aqueous medium. They result from the acid hydrolysis of cellulose fibers, where acid dissolves the amorphous domains while highly crystalline domains, more resistant to the acidic attack, remain intact. These nanoparticles are called nanocrystals or whiskers. For instance, Figure 1 shows the morphology of cellulose whiskers extracted from ramie fibers. It is also possible to omit the acid hydrolysis step and to submit the cellulosic fibers to a mechanical treatment involving high shearing forces. Ensuing nanoparticles, called cellulose microfibrils or microfibrillated cellulose, resulting from this treatment occur as very long flexible nanoparticles or filaments. These nanoparticles represent a high added-value product for forestry and paper industries. In the case of starch nanocrystals, obtained following the same procedure as for whiskers, resulting nanoparticles occur as platelets.

Transmission electron micrograph of ramie cellulose nanocrystals
Figure 1 - Transmission electron micrograph of ramie cellulose nanocrystals

New bio-nanofiller sources

Cellulose whiskers

We were interested in the preparation of cellulose nanocrystals or whiskers from different sources. Indeed, depending on the source of cellulose, it is possible to obtain stiff rod-like nanoparticles with different geometrical characteristics. The acid hydrolysis conditions also affect the geometrical characteristics of the nanoparticles. The average length ranges between 100 nm and 1 μm and the diameter ranges between few nanometers to 20 nm. These dimensions impact the aspect ratio of the nanoparticles (ratio of the length to the diameter), and consequently the percolation threshold. We have also proved that a correlation exists between the aspect ratio and the stiffness of a film obtained from the evaporation of an aqueous suspension of cellulose whiskers.

Cellulose microfibrils

Cellulose whiskers are interesting nanoparticles because of their well-defined form which allows theoretical studies. From a practical point of view, cellulose microfibrils are certainly more accessible reinforcing elements in term of available amounts. They result from a defibrillation mechanical treatment of cellulosic fibers. We were interested in comparing the mechanical reinforcing effect obtained with cellulose whiskers and microfibrils extracted from the same source, in the case of sisal leafs and rachis of the date palm tree.

Starch nanocrystals

Starch nanocrystals are obtained by acid hydrolysis of native starch granules. The ensuing nanoparticles occur as nanometric scale platelets. We continue the studies engaged with nanocrystals extracted from waxy maize: we also explore other sources to study the impact of the botanical source on the morphology and dimensions of the nanoparticles in the framework of an european project.

Nanocomposites processing and properties

Casting-evaporation

Polysaccharide nanoparticles are obtained as aqueous suspensions. The stability of these suspensions is due to sulphate groups resulting from the acid hydrolysis treatment when sulphuric acid is used, or to residual hemicelluloses at the surface of the nanoparticles in the case of microfibrils. The homogeneous dispersion of nanoparticles in a polymeric matrix is a key step that conditions the end-use properties of the material.

Because of the stability of aqueous suspensions of these nanoparticles, water is naturally a good medium for the processing of nanocomposite materials. Therefore, we were interested to hydrosoluble polymers, such as for instance polyvinyl alcohol (PVA). The main interest in this polymer is that copolymers of PVA and polyvinyl acetate are easily available and that one can change the affinity of the polymeric matrix with the surface of the nanoparticles. Nanocomposite films were simply obtained by blending the suspension with the solution and evaporating water. A first alternative consists in using a polymeric matrix in the form of an aqueous suspension, i.e. latex. Natural rubber is a good candidate because besides its natural origin, it presents the advantage to have a glass temperature much lower than the room temperature which facilitates the processing and to be fully amorphous. The nanocomposite films are simply obtained by blending the two suspensions and evaporating water.

A second alternative consists in using a non aqueous liquid medium. It means that nanoparticles should be dispersed in an organic medium which nature depends on the matrix to be used. Successive solvent exchanges followed by centrifugations can be used. It is generally necessary to chemically modify the surface of the nanoparticles to increase their hydrophobic character. This strategy is supported by competences reported in Topic 2. We can also use coupling agents that can make the surface of the nanoparticles more hydrophobic.

A more innovative technique consists in grafting large chains (oligomers or macromolecules) on the surface of the nanoparticles. The grafting agents should present a reactive moiety which reacts with hydroxyl groups of cellulose and a long "compatibilizing" tail. The main interest in this approach is that it allows the formation of a continuous interphase between the filler and the polymer matrix phase that can improve interfacial adhesion. Moreover, entanglements between grafted and ungrafted polymer chains are expected to occur if the molar weight is high enough.

There are basically two approaches, called "grafting onto" and "grafting from". In the case of cellulose whiskers and polycaprolactone (PCL), both approaches have been explored. We also investigated the influence of the length of the grafted chain in the case of fatty aliphatic chains. In any case, the crystallization of surface grafted chains was evidenced. For starch nanocrystals, we observed that the nature of the grafted chains had an incidence on the capability of grafted chains to crystallize. When the chains display some affinity with the surface of the nanoparticles as for poly(ethylene glycol), the crystallization does not occur, whereas in the case of fatty aliphatic chains it is clearly observed [Figure 2].

It was shown by the comparison between cellulose whiskers and microfibrils that the reinforcing effect was much higher with the latter. This effect was ascribed to the possibility of entanglements of the microfibrils and to the presence of residual hemicelluloses at the surface that allow a better natural compatibilisation between the hydrophilic reinforcing phase and the hydrophobic matrix.

X-ray diffraction patterns of (a) unmodified, (b) PEGME-TDI- 
   and (c) stearate-modified starch nanocrystals
Figure 2 - X-ray diffraction patterns of (a) unmodified, (b) PEGME-TDI-
and (c) stearate-modified starch nanocrystals

Extrusion

Although it is a classical processing method for polymers, extrusion has been only slightly used for the processing of polysaccharide nanocrystals reinforced nanocomposite materials up to now. It is essentially ascribed to their strong tendency to aggregate when dried. However, this effect can be limited if the filler-matrix interactions are high enough. We have seen for instance that this technique can be used to obtain nanocomposite materials consisting of a poly(ethylene oxide) (PEO) matrix and cellulose nanoparticles (whiskers or microfibrils). The potential application of this system was polymer electrolytes for lithium batteries that were already studied but only for films obtained by casting-evaporation. For some highly hydrophobic polymeric matrices, as for instance polyethylene, the surface chemical modification of the nanoparticles is required. We have shown that it was possible to homogeneously disperse cellulose whiskers modified with aliphatic chains in polyethylene using extrusion [Figure 3]. However, the mechanical reinforcing effect was disappointing because when increasing the filler-matrix compatibility, the possibility of filler-filler interactions was reduced. Nevertheless, the ductility of the material was strongly improved.

Photographs of neat LDPE film and ramie cellulose whiskers 
   based nanocomposite films reinforced with 10 wt% of unmodified 
   whiskers and stearoyl chloride-modified whiskers
Figure 3 - Photographs of neat LDPE film and ramie cellulose whiskers
based nanocomposite films reinforced with 10 wt% of unmodified
whiskers and stearoyl chloride-modified whiskers

Conclusions

Nanoparticles can be extracted from biomass. The morphology of these nanoparticles depends on the origin of the natural substrate. These nanoparticles can be used to prepare high performances nanocomposite materials. However, the processing conditions have to be adjusted depending on the nature of the polymeric matrix used.

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