This study aimed at understanding the adhesion between wood fibers and thermoplastics. Direct applications include the development of better wood fiber composites and better paper coatings. The objectives of the study were two-fold. First, to quantify the effects of surface treatments on the surface properties, and, second, to determine if adhesion can be described in terms of surface properties. A model consisting of amorphous cellulose spherical beads was used to eliminate the effects of morphology, composition, and fiber size and orientation; adhesion was studied only in terms of surface properties.
The surface of the cellulose beads was modified by blending, by coating or by chemical surface reaction. Surface modification by blending was achieved by dissolving cellulose along with another polymer (cellulose propionate) and by beading the mixed solution. An alternative consisted of coating the beads with a thin layer of polymer, such as poly(4-vinyl-pyridine-co-styrene). Finally the surface was also modified by grafting functional groups or polymer segments onto the hydroxyl groups of cellulose. A thin layer of cellulose derivative, such as cellulose trifluoroethoxyacetate or cellulose laurate was produced on the bead surface. Polystyrene and polypropylene segments were grafted onto cellulose to create an interphase.
The surface properties of the cellulose beads were then fully characterized. The surface composition was analyzed by X-ray Photoelectron Spectroscopy (XPS), and the morphology was investigated through Scanning Electron Microscopy. Inverse Gas Chromatography (IGC) was used to measure the surfaces' energetics. Two kinds of probes were used: alkanes to measure the dispersive component of adhesion, and acid/base probes to quantify the specific properties. Composites having variable bead content were made by injection molding. The adhesion between the cellulose beads and the thermoplastics were measured by tensile testing and by DMTA using the modified Nielsen Model with the damping factor (tan δ).
The surface energy of cellulose was found to depend mostly on the presence and concentration of free hydroxyl groups on the surface. For low degrees of substitution (DS), how these OH groups are replaced by modification, whether by fatty acid type substituents or by fluorine-containing groups, is essentially irrelevant for surface characteristics. The dispersive component of the surface energy (γs<sup>D</sup>) declined with DS, almost irrespective of substituent type. The acidic character of the cellulose surface is attributed mainly to the presence of hydroxy groups. It was furthermore established that, while wetting is a necessary condition, it is in itself insufficient for achieving good adhesion and adequate composite strength characteristics. The mechanical properties of polyethylene, polypropylene and polystyrene all decreased as cellulose beads were added in increasing amounts. It was found that improved cellulose fiber-reinforced composite performance requires the development of an interphase, such as by grafting polymer segments onto the cellulose surface which are capable of entanglement with the thermoplastic polymer in the melt. Maleic anhydride/polypropylene copolymers were found to be efficient coupling agents that better transmit stress when their molecular size increased. Adsorption of poly(4-vinylpyridine-co-styrene) [basic] by the cellulose beads [acidic] resulted in completely coated surfaces. However, strength differences between composites, with coated and uncoated beads, were insignificant probably owing to the large bead sizes used. / Ph. D.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/106881 |
| Date | January 1993 |
| Creators | Garnier, Gil |
| Contributors | Chemical Engineering |
| Publisher | Virginia Polytechnic Institute and State University |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation, Text |
| Format | xiii, 338 leaves, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 29179263 |
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