Characterization of vibration-welded wood and natural fiber composites
Date
2022-05
Authors
Covelli, Curtis R
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Grewell, David A
Raman, D Raj
Schmidt-Rohr, Klaus
Kraus, George A
Brumm, Thomas J
Moore, Kenneth J
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Abstract
The focus of this dissertation was to understand the process and property relationships of vibration-welded wood and natural fiber composites through their characterization. The work was divided into three research focuses according to material type. Firstly, wood (a natural composite of cellulose fiber in a hemicellulose-lignin matrix) weld morphology was analyzed using nuclear magnetic resonance (NMR) spectroscopy to determine the molecular and chemical changes in the wood after vibration welding. Secondly, the weld strength of agave fiber (AF) reinforced thermoplastic composites was analyzed using a full factorial experimental design with three weld parameters, in which the neat thermoplastic resin and the contribution of the natural fiber to the weld were examined. Finally, the welding of pine, maple, and bamboo pulpboard was studied. Maple was selected as a hardwood, pine as a common softwood, and bamboo pulpboard was included as a midpoint between the structure of wood and a fiber-impregnated thermoplastic resin.
The first research focus centered on structural changes of welded wood, analyzed through solid-state NMR. Molecular structural changes in the weld of pine and maple woods as well as maple-wood flash were investigated using quantitative solid-state 13C NMR, supplemented by 13C and 1H relaxation-based spectral editing. Changes in the NMR peak positions documented the transformation of crystalline cellulose from I to I. Between 11% and 19% of non-crystalline cellulose and acetylated hemicellulose was transformed chemically into, for example, aromatic C-CH2-C and C=O moieties. This cellulose and hemicellulose transformation accounts for approximately 30% of the wood. In the pine weld, the observed increase in aromaticity was the result of O-containing aromatic rings forming near cellulose and not lignin. This was seen in 1H inversion recovery with 13C detection, and is consistent with the increase in aromatic C-O signal intensity. A similar observation was made in the maple weld, although the analysis was complicated because 1H relaxation of lignin was inhomogeneous. The flash showed evidence of additional pyrolytic transformations. These results suggest that wood behaves in a manner similar to thermoplastic materials during vibration welding.
For the second focus, injection-molded AF filled bio-composites of polypropylene (PP) and polylactic acid/polybutyl succinate (PLA/PBS) resins were the materials used for welding. A vibrational welder (Branson Mini II) was used, and a factorial design with the parameters of weld pressure, amplitude, and weld time was used to study the effects of these parameters on lap shear weld strength. A custom fixture was used to secure and weld injection-molded ASTM type I tensile bars. The welds were analyzed to gain insight into the mechanisms of the welding, which showed a proportional relationship between welded amplitude and weld strength, and the effect of amplitude was greater than that of pressure, which was greater than that of weld time. Among higher parameter combinations, strength was inversely proportional to the independent parameters, possibly due to the migration of fibers at the interface for both the PP/AF and PLA/PBS/AF formulations.
The final focus investigated the factors affecting the welding of pine, maple, and bamboo pulpboard with a commercially available vibration welding system typically used for welding plastics. Weld pressure, amplitude, and weld time were all varied to determine their effects on lap shear weld strength. Strength testing was conducted using a universal testing machine. The morphology of the weld zone was also analyzed to gain insight into the welding mechanics. The highest strength of pine samples was 8.4 MPa, while maple was approximately 35% stronger and had a smaller standard error, and bamboo pulpboard seemed to weld in a similar fashion to wood. Additionally, pulpboard weld strength was primarily dependent on weld pressure.
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dissertation