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V - Converting Biomaterials Packaging

V - 3 - Recent advances on surface chemical modification of cellulose fibres

Several approaches of chemical modification of cellulose fibre surface have been studied, in order to reduce their hydrophilic character and to improve the strength of their adhesion to the matrix in composite materials.

Relevant advances were achieved recently in different modification strategies, namely:

The characterisation of the modified surfaces was performed using elemental analysis, contact angle measurements, Scanning Electron Microscopy, CPMAS 13C-NMR, FTIR and X-ray photoelectron spectroscopy. The present paper reports the most relevant advances in the field of surface chemical modification of cellulose fibres but also concerning new bionanoelements like cellulose whiskers and microfibrillated cellulose.

Introduction

The use of natural cellulose fibres as reinforcing elements in macromolecular composite materials is a growing topic of investigation, particularly in view of replacing, at least partly, glass fibre-based composites, which cannot be recycled at the end of their life cycle. Nanoscale cellulose elements emphasize this reinforcement effect. However lignocellulosic materials possess major limitations when used in the field of composite materials, since they are highly polar and hydrophilic. As a consequence, they are poorly compatible with commonly used non-polar matrices and they are subjected to a loss of mechanical properties upon atmospheric moisture adsorption. In order to avoid these drawbacks, they are systematically submitted to specific surface modifications in order to provide them with an efficient hydrophobic barrier and to compatibilize or even linked their surface with that of a non-polar polymer matrix. In this context, the chemical moieties exploited are superficial hydroxy functions, which can be converted into esters, ethers, urethanes or siloxanes functions.

Surface treatments of cellulose

Plasma

The treatment of additive-free handsheet paper samples with cold-plasma showed that cellulose can be chemically linked with either reactive silane coupling agents (vinyl trimethoxysilane, VTS, and γ -methacrylopropyl trimethoxysilane, MPS) or natural products (myrcene, My, and limonene, LM). Contact angle measurement and X-ray Photoelectron Spetroscopy (XPS) were used to ascertain the occurrence of the grafting. In fact, the contact angle value of a drop of water deposited at the surface of paper increased from 30° for unmodified substrate to more than 100° for VTS-, MPS-, LM- and My-treated samples [Figure 1].

Contact angle of a water droplet deposited on the surface of paper before and after treatment
Figure 1 - Contact angle of a water droplet deposited
on the surface of paper before and after treatment

Moreover, the treated surfaces became totally non polar, as their polar contribution to the surface energy decreased from about 23 mJ/m2, for pristine samples, to practically zero for all the treated surfaces. For VTS- and MPS-treated samples, the XPS spectra showed the appearance of two new peaks at 102 and 150 eV, relative to the presence of Si atoms and a substantial increase in Cl signal, attributed to the enrichment of the surface by C–H moieties borne by the silanes. SEM confirms the presence of the silane, both in the bulk and at the surface of the treated samples. The treated surfaces displayed water-barrier properties, since the penetration of the liquid was reduced significantly. The XPS spectra showed that the modification with LM and My gave rise to a very significant change in the O/C ratio, as well as to the intensity of the C1 peak assigned to aliphatic carbon sequences [Figure 2].

C1s deconvolution XPS spectra of virgin and PCL-treated cellulosic substrates with monoactivated polymer
Figure 2 - C1s deconvolution XPS spectra of virgin
and PCL-treated cellulosic substrates with monoactivated polymer

Chemical grafting with mono-activated polymer

First idea in compatibilisation with matrix is to create a “bridge” between cellulose fibres and matrices by using a bi-functional molecule which will react with cellulose hydroxyls on one hand and reactive groups from matrices on the other hand. For example, a recent study use grafting agents like: pyromellitic dianhydride (PMDA), benzophenone-3,3′,4,4′ -tetracarboxylic dianhydride (BPDA), 1,4-phenylene diisocyanate (PPDI), methylene-bisdiphenyl diisocyanate (MDI).

In all cases stiff molecules were used with the aim of ensuring the reaction of only one of the functionalities with the cellulose surface, leaving the remaining moiety to react with the polymer matrix during composite processing to provide a covalent linkage between the matrix and the reinforcing elements, thus enabling perfect stress transfer. Another strategy consists in grafting directly the polymer chains on the fibres. In this case, it seems possible to have sufficient polymer grafted to obtain a continuous material only by melting it. The idea is to create an activated polymer (more oligomer) which will react with cellulose.

In our case we observed recently that cellulose fibres can be successfully modified with functionalized oligoethers thanks to an innovative process based on isocyanates reactivity. Two cellulosic substrates (Whatman paper (WP) and wood fibres) were chemically modified. Poly(ethylene), poly(propylene) and poly(tetrahydrofuran) glycols with different lengths were converted to mono-NCO-terminating macromolecules, which were monitored by FTIR spectroscopy. The prepared macromolecular grafts were then coupled with cellulose surface and the resulting treated substrates fully characterized by contact angle measurements, elemental analysis, and XPS.

Thus, all the techniques implemented showed clear cut evidences about the occurrence of the grafting, namely:

The same idea has been successfully achieved by using poly-caprolactone (PCL) as grafted polymer. It is sufficient to modify strongly the surface energy but this limitation of the amount of grafting to only few percents had to be overcome. Therefore, “click chemistry” has been tested.

Chemical grafting by click chemistry

Very recently, a new approach of grafting cellulose surface fibres by polycaprolactone macromolecular chains in heterogeneous conditions via click-chemistry was reported. Cellulose esters were prepared by reacting Avicel with undecynoic acid, in order to prepare a cellulose substrate bearing multiple C≡C-terminated hairs. The modified Avicel samples were first characterised by FTIR and XPS spectroscopy and elemental analyses and showed that the grafting had occurred. In parallel, polycaprolactone-diol (PCL) was converted into an azidoderivative and the ensuing products were characterised by FTIR and 13C-NMR spectroscopy. Both methods confirmed the success of such a modification.

Finally, cellulose esters were reacted with azido-PCL grafts, in heterogeneous conditions, through “click chemistry”. The resulting modified cellulose substrates were characterized by the techniques mentioned above. All the techniques confirmed that the grafting has occurred efficiently, since a weight gain of about 20% was achieved.

Direct silanol-cellulose condensation

Much more recently, the surface of model cellulose fibres, Avicel (AV), as well as that of Whatman paper (WP) was chemically modified with TFPS (3,3,3-trifluoropropyl trimethoxysilane) and PFOS (1H,1H,2H,2H,perfluorooctyl trimethoxysilane) [Figure 3]. After modification, the ensuing fibres were submitted to soxhlet extraction, in order to remove all physically adsorbed unbounded molecules. The occurrence of the grafting was confirmed by the presence of silicon and fluorine atoms detected by elemental analysis, X-ray photoelectron spectroscopy and Scanning Electron Microscopy (SEM). The contact angle measurements showed that, after grafting, the cellulose surface became totally hydrophobic.

SEM micrographs of the surface of (a) virgins WP, (b) TFPS- and (c) PFOS-treated WP
Figure 3 - SEM micrographs of the surface of (a) virgins WP,
(b) TFPS- and (c) PFOS-treated WP

The C1s spectra showed that the modification of cellulose with TFPS and PFOS displayed some substantial changes, namely:

AV seems to be systematically more reactive, which can be related to its higher specific surface area compared with PW. The SEM micrographs and the EDS mappings were performed. Fluorine and silicon atoms were localized at the surface and throughout the thickness of the cellulosic substrates. In all cases, both fluorine and silicon seemed to be distributed homogeneously, both at the surface and throughout the thickness of the materials.

Kinetics of a hydrolysis/solvolysis reaction of silane coupling agents

Different N-bearing alkoxy-silane coupling agents, namely: 3-cyanopropyl triethoxy silane (CPES), triethoxy-3-(2-imidazolin-1-yl) propyl silane (IZPES), and amino silanes, 3-aminopropyl triethoxy silane (APES), 3-aminopropyl trimethoxy silane (APMS), 3 (2-aminoethylamino)propyl trimethoxysilane (DAMS), 3-[2-(2-aminoethylamino) ethylamino]propyl trimethoxysilane (TAMS), 4-amino-3,3-dibutyl trimethoxy silane (ADBMS) and trimethoxy [3-(phenylamino) propyl] silane (PAPMS) were hydrolyzed in an ethanol/water 80/20 (w/w) solutions under acidic conditions. The hydrolysis kinetics was followed in situ by 1H-, 13C- and 29Si-NMR spectroscopy and showed that acidic conditions were the most suitable to enhance the formation of silanol and to slow down the self condensation reactions of the hydrolyzed functions. 29Si NMR spectroscopy revealed the formation of intermediate species, particularly the solvolysis of γ -amino silanes by alcoholic solvent exchange reaction.

More recently, other three alkoxy-silane coupling agents, namely: triethoxy vinyl silane (VES), trimethoxy (2-phenylethyl) silane (PEMS) and trimethoxy (7-octen-1-yl) silane (OEMS), were tested in the same conditions. Here also, the reaction was followed by 1H-, 13C- and 29Si-NMR spectroscopy. 29Si NMR in situ allowed the determination of the intermediate species, as a function of the reaction time. It was established that the most reactive species (T0H) were formed and reached their maximum amounts after 2, 3 and 4 hours, for OEMS, PEMS and VES, respectively. The values of the maximum amounts of such species were around 60, 80 and 90%, for PEMS, OEMS and VES, respectively. It was also found that the non-reactive T3 species never appeared, suggesting that the reaction medium arising from the pre-hydrolysis of the silanes still possessed reactive silanes even after 48 hours reaction, under acidic conditions.

Influence of nanosized cellulose elements in fatty acid grafting

Cellulose nanocrystals (or whiskers) and microfibrillated cellulose (MFC) were successfully obtained from sisal fibers and modified with n-octadecyl isocyanate (C18H37NCO) using two different methods with one innovative that consists in a insitu solvent exchange procedure. The surface chemical modification was characterized by elemental analysis, as well as FTIR and XPS spectroscopies. The crystalline structure of both unmodified and modified nanoparticles was investigated through X-ray diffraction measurements. It was shown that the efficiency of the chemical modification is strongly dependent on the nature of the nanoparticle with explanation linked to specific area, ability of peeling and solvent dispersion. The surface chemical modification with n-octadecyl isocyanate allows dispersion of the nanoparticles in organic solvents and processing of nanocomposite films from a casting/evaporation technique for a broad range of polymeric matrices.

Conclusions

Cellulose macromolecules can be grafted successfully by natural polymerizable molecules or silane coupling agents and using a solvent-free process, at room temperature. Such a treatment can be considered as a promising new approach, particularly in papermaking and textile applications, where the hydrophobisation of the tissues or paper-based packaging materials is an extremely important issue.

The hydrolysis of different alkoxysilanes was also studied in order to establish the optimal reaction conditions yielding the highest amount of reactive species. In fact, this family of reagents is known to undergo easy hydrolysis, but presents a major drawback with respect to their self-condensation reaction. The latter is undesirable in the context of cellulose grafting.

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