Surface and In-Depth Modification of Cellulose Fibers - American

Chapter 6

Surface and In-Depth Modification of Cellulose Fibers 1

Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: April 16, 2007 | doi: 10.1021/bk-2007-0954.ch006

Alessandro Gandini and Mohamed Naceur Belgacem

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CICECO and Chemistry Department, University of Aveiro, 3810-193 Aveiro, Portugal Laboratoire de Génie des Procédés Papetiers, UMR 5518, École Française de Papeterie et des Industries Graphiques (INPG), BP65, 38402 Saint Martin d'Hères, France

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This chapter is divided into three parts, according to the application envisaged for the modified fibers, namely (i) as reinforcing elements in macromolecular composite materials; (ii) in wood densification and protection; and (iii) for trapping organic pollutants. The major emphasis in the first part is devoted to the controversial aspects related to the interactions between cellulose and siloxanes. The second part illustrates our approach through the use of bifunctional coupling agents and the subsequent grafting of the densifying polymer. The third part shows how admicelles or aliphatic brushes, built around the fibers, play a useful role in capturing organic impurities from aqueous media.

Cellulose is the most abundant natural polymer on earth, found mostly in vegetal biomass, where it is produced by photosynthesis in the form of semicrystalline fibers. The exploitation of this ubiquitous and inexpensive renewable resource remains a very important issue today, not only for the production of high volume commodities like textiles and paper, but also for novel value-added materials. Such materials may take advantage of the specific properties associated with the cellulose fibers (low density, good mechanical © 2007 American Chemical Society

In Materials, Chemicals, and Energy from Forest Biomass; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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94 performances, biodegradability, etc.) and the versatility toward partial or total chemical modification. The syntheses of cellulose esters, ethers and other derivatives, some of which are well-established industrial processes, involve the destruction of the fibrous morphology and its transformation into thermoplastic materials soluble in organic solvents (7-5). Recently, similar chemical processes were applied to cellulose, but limiting their impact to the macromolecular layers which constitute the fiber surface. The main purpose of this novel approach is to conserve the fiber integrity, and thus its mechanical properties, but to modify some specific features associated with its surface. One major driving force for the vast amount of research conducted on this topic (4-8) is the growing interest in composite materials in which a polymeric matrix is reinforced by cellulose fibers, replacing glass counterparts. The driving forces behind this are: (i) ecological considerations including the renewable character of cellulose and its biodegradability, (ii) economic factors, including, low cost and abundant fiber supply, lower density (which implies lower fuel consumption when the composites are transported) and lower abrasion resistance during processing; (iii) satisfactory mechanical performance of the composites. The fiber surface treatment fulfills two roles, viz. the improvement of the interfacial adhesion with the matrix and the reduction of moisture uptake by the fibers. On another front, the in situ surface chemical modification of lignocellulosic fibers of wood (9) can bring about notable improvements related to hydrophobization and resistance to biocides. Due to the demonstrated efficiency of cellulose as the stationary phase in gas-liquid chromatography, we recently extended this feature by using appropriately modified cellulose substrates to trap organic pollutants from contaminated waters (70-72).

Modification of Cellulose Fibers for Use as Reinforcing Elements in Macromolecular Matrices Siloxane coupling agents are commercial compounds commonly used to modify the surface of glass fibers by reacting with their O H groups directly or after (partial) hydrolysis, as shown in Scheme 1. Since cellulose fibers also possess an OH-rich surface, it was thought that these reagents could be suitable for similar modifications. However, a fundamental study aimed at establishing the reactions occurring between different silane coupling agents and the surface of cellulose (75) showed that reaction 1 (Scheme 1) did not take place with six silane coupling agents, namely vinyltrimetoxy-(VS), γ-methacryloxypropyltrimetoxy- (MPS), cyanoethyltrimetoxy- (CES), γ-aminopropyltrimethoxy- (APS), octyltriethoxy- (OS) and glycidilpropyltrimethoxy-silane (GPS). In fact, these reagents were found to



95 condense with cellulose O H groups only if they had previously been hydrolyzed, i.e. only through reaction 2 (Scheme 1). This finding was attributed to the more nucleophilic character of the cellulosic OH's, compared with their counterparts present in glass. In other words, the siloxane moiety was not cleaved if the O H group attached to the solid substrate was not sufficiently acidic, whereas the same substrate underwent condensation reactions between its O H groups and silanol moieties (75). This was confirmed by the observation that lignin reacts with the unhydrolyzed siloxanes, through its acidic phenolic O H groups (75). In a subsequent investigation, we studied the hydrolysis kinetics M P S , A P S and γ-diethylenetriaminopropyl trimethoxy silane (TAS) (14).

+ H 0 2

Scheme 1. Condensation reactions between pristine and hydrolyzed siloxanes and glass surface OH groups

VS

MPS

N - G


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OS

GPS

TAS

The rate of these reactions, carried out in an 80/20 (w/w) ethanol(or methanol)/water solution, increased in the order MPS

Surface and In-Depth Modification of Cellulose Fibers - American

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