Browsing by Author "Frazier, Charles E."

Now showing 1 - 20 of 75
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    Adhesion Fundamentals in Spotted Gum (Corymbia citriodora)
    Burch, Coleman Patrick (Virginia Tech, 2015-12-23)
    The goal of this project was to advance adhesion science and technology related to the Australian hardwood spotted gum (Corymbia citriodora). Plantation-grown spotted gum exhibits poor adhesion properties in comparison with similar woods, such as Gympie messmate (Eucalyptus cloeziana). To better understand adhesion differences between these two woods, this research compared and contrasted the surface chemistries of plantation-grown spotted gum and Gympie messmate with a particular focus on sensitivity to thermal deactivation. Wetting measurements were performed using the sessile drop method. Initial and equilibrium contact angles, time-dependent wetting, and surface energy were determined. Time-dependent wetting and equilibrium contact angles were most informative. Initial contact angles and surface energy calculated with them were misleading and often generated anomalous results. Heating water-saturated wood to mild surface temperatures (105 deg C, directly after evaporative cooling) severely deactivated spotted gum but not Gympie messmate. This suggests conventional kiln drying appears unsuitable for spotted gum while amenable for Gympie messmate. Spotted gum likely requires non-aqueous, low surface tension adhesives or aqueous adhesives formulated with surface active wetting agents. Water-saturation (followed by room-temperature vacuum drying) substantially altered the surface chemistries of both woods, making them more hydrophilic. Consequently, the question was raised of whether a water-spray onto the wood surface prior to adhesive application could improve bonding. If so, this simple, industrially-feasible treatment could prove very beneficial to the wood composites industry. Water-saturation also revealed differences in the two wood's water permeability, which has implications for adhesive penetration and wood drying and may additionally help explain adhesion differences. Analysis of the plantation-grown heartwood (inner, middle, and outer heartwood regions) revealed significant wetting differences on spotted gum with only minor differences on Gympie messmate. The Australian woods were compared to two North American woods-loblolly pine (Pinus taeda) and Douglas-fir (Pseudotsuga menziesii). Examining water wetting measurements, the Australian and North American woods exhibited some interesting similarities. However, methylene iodide wetting measurements revealed that the Australian woods were quite different from the North American samples studied here.
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    Adhesive Bonding of Low Moisture Hickory Veneer with Soy-based Adhesive
    Wykle, Cody James (Virginia Tech, 2019-06-10)
    Low moisture veneer and regions of sapwood within hickory engineered wood flooring bonded with soy-flour adhesive are thought to be factors leading to potential performance deficiencies. The goal of this research was to gain a broader understanding of bonding low moisture hickory veneer with soy-based adhesive. Soyad® is of particular interest due to its novel cross-linking chemistry. Impacts of moisture content and wood region (heartwood versus sapwood) were analyzed with dry and wet shear bond strength tests, measurement of percent wood failure, lathe check characterization, and adhesive bondline thickness and penetration depth measurement. Impact of wood region and type (hickory versus red oak) was assessed by comparing wood buffering capacity and delamination following three-cycle water soaking. Dry and wet shear strength values met expectations for engineered wood flooring yet percentage wood failure results were uniformly very low for all combinations of moisture levels and wood regions. In contrast, delamination following wet and dry cycling was minor and within minimum requirements for all specimens tested. The influence of moisture level, wood region and type were inconsistent; statistically significant relationships were not evident within the moisture range studied. However, different wood regions and types exhibited differing veneer buffering capacities that had potential to interfere with pH requirements of Soyad®. Additional study of buffering capacity and resin cure is recommended to determine the significance of the buffering capacity results found in this study.
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    Advancing characterization techniques for structure-property determination of in-situ lignocelluloses
    Chowdhury, Sudip (Virginia Tech, 2011-07-21)
    The global progression towards sustainable energy, materials and chemicals requires novel and improved analytical tools to understand and optimize lignocellulosic biomass utilization. In an effort to advance lignocellulose characterization, gain insights into biomass processing, and obtain novel perspectives on cell wall ultrastructure, this study utilizes three principal polymer characterization techniques, namely compressive-torsion dynamic mechanical analysis (DMA), deuterium quadrupolar nuclear magnetic resonance (2H NMR) and rheo-infrared spectroscopy. A novel parallel-plate compressive-torsion DMA protocol is developed to analyze very small solvent-plasticized biomass specimens with or without mechanical integrity. The benefits and limitations of this technique are demonstrated by comparing it to a conventional tensile-torsion DMA while analyzing various solvent-plasticized lignocelluloses. The rheology of wood in various organic solvents is studied through dynamic thermal scans, Time/temperature superposition (TTS) and fragility analysis. Plasticizing solvents and wood grain orientation significantly affected the lignin glass-transition temperature. Dynamic TTS reveals that while all storage modulus data shift smoothly, the thermorheological complexity of solvent-plasticized wood becomes evident in loss component master curves. It is argued that the plasticized lignocellulose TTS is insightful and potentially useful, although it fails to satisfy the classic TTS validity criteria. Subsequently, it is justified that the fragility analysis is a better suited treatment than the WLF model to investigate cooperative segmental motions of plasticized wood. Deuterium quadrupolar NMR reveals a new perspective on the orientation of amorphous wood polymers and two distinct amorphous polymer domains: a highly oriented phase in the S2 layer of the secondary cell wall and an isotropic phase postulated to occur in the compound middle lamella (CML). If the origin of the isotropic phase is confirmed to arise from the CML, then this technique provides a way to independently investigate the morphology and phase dynamics of CML and S2 in an intact tissue, and should bring novel insights into deconstructive strategies specific to the oriented and unoriented domains. Finally the effects of a wood-adhesion promoter (hydroxymethyl resorcinol, HMR) on in-situ wood polymers are studied to elucidate the still unresolved HMR-lignocellulose interactions. DMA, creep-TTS and 2H NMR reveal that HMR increases the crosslink density and restricts the mobility of wood amorphous phase. Rheo-IR spectroscopy shows that the molecular stress-transfer mechanism is altered within the wood cell wall.
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    Aspects of Wood Adhesion: Applications of 13C CP/MAS NMR and Fracture Testing
    Schmidt, Robert G. (Virginia Tech, 1998-01-28)
    Phenol Formaldehyde (PF) and polymeric isocyanate (pMDI) are the two main types of adhesives used in the production of structural wood-based composites. Much is unknown about various aspects of adhesion between these two types of resins and wood. The present research describes the development of techniques which will permit an enhanced understanding of 1.) the extent of cure of PF within a wood based composite, 2.) the scale of molecular level interactions between PF and pMDI and wood, 3.) mechanical performance and durability of wood-adhesive bonds. Correlations were established between conventional methods of characterization of neat PF (thermomechanical analysis, swelling studies) and measurements made using 13C CP/MAS NMR. These correlations were then utilized to characterize PF cured in the presence of wood. The use of 13C labeled PF allowed estimates of relative degrees of resin conversion to be made. The use of 13C and deuterium labeled PF allowed qualitative estimates of resin molecular rigidity to be made. The scale of molecular level interactions between PF and pMDI and wood was probed using NMR relaxation experiments. Evidence was shown to suggest the formation of an interpenetrating polymer network (IPN) morphology existing at both types of wood-resin interphases. The formation of the IPN morphology was strongly influenced by resin molecular weight, cure temperature and the presence of solvent. A new test geometry for the evaluation of the fracture toughness of wood-adhesive bonds was developed. Consistent and reliable results were obtained. It was found that low molecular weight PF possessed enhanced durability over high molecular weight.
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    Bio-inspired Cellulose Nanocomposites
    Pillai, Karthik (Virginia Tech, 2011-04-26)
    Natural composites like wood are scale-integrated structures that range from molecular to the macroscopic scale. Inspired by this design, layer-by-layer (LbL) deposition technique was used to create lignocellulosic composites from isolated wood polymers namely cellulose and lignin, with a lamellar architecture. In the first phase of the study, adsorption of alkali lignin onto cationic surfaces was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). Complete coverage of the cationic surface with alkali lignin occured at low solution concentration; large affinity coefficients were calculated for this system at differing pH levels. Adsorption studies with organosolv lignin in an organic solvent, and spectroscopic analysis of mixtures of cationic polymer with alkali lignin revealed a non-covalent interaction. The work demonstrated how noncovalent interactions could be exploited to molecular organize thin polyphenolic biopolymers on cationic surfaces. The second phase of the study examined the adsorption steps during the LbL assembly process to create novel lignocellulosic composites. LbL assembly was carried out using oxidized nanocellulose (NC) and lignin, along with a cationic polymer poly(diallyldimethylammonium chloride) (PDDA). QCM-D was used to follow the sequential adsorption process of the three different polymers. Two viscoelastic models, namely Johannsmann and Voigt, were respectively used to calculate the areal mass and thickness of the adsorbed layers. Atomic force microscopy studies showed a complete coverage of the surface with lignin in all the disposition cycles, however, surface coverage with NC was seen to increase with the number of layers. Free-standing composite films were obtained when the LbL process was carried out for 250 deposition cycles (500 bilayers) on a cellulose acetate substrate, following the dissolution of the substrate in acetone. Scanning electron microscopy of the cryo-fractured cross-sections showed a lamellar structure, and the thickness per adsorption cycle was estimated to be 17 nm. The third phase of the study investigated the effect of LbL ordering of the polymers versus a cast film composed of a blended mixture of the polymers, using dynamic mechanical analysis. A tan ï ¤ peak was observed in the 30 – 40 ºC region for both films, which was observed in the neat NC film. Heating of the samples under a compressive force produced opposite effects in the films, as the LbL films exhibited swelling, whereas the cast films showed densification. The apparent activation energy of this transition (65 – 80 kJ mol-1) in cast films, calculated based on the Arrhenius equation was found to be coincident to those reported for the ï ¢ transition of amorphous cellulose. The peak was seen to disappear in case of LbL films in the second heat, whereas it was recurring in case of cast films of the blended mixture, and neat NC films. Altogether, the together the work details a novel path to integrate an organized lignin and cellulose molecular structure, albeit modified from their native form, into a three-dimensional composite material.
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    Bioactive Cellulose Nanocrystal Reinforced 3D Printable Poly(epsilon-caprolactone) Nanocomposite for Bone Tissue Engineering
    Hong, Jung Ki (Virginia Tech, 2015-05-07)
    Polymeric bone scaffolds are a promising tissue engineering approach for the repair of critical-size bone defects. Porous three-dimensional (3D) scaffolds play an essential role as templates to guide new tissue formation. However, there are critical challenges arising from the poor mechanical properties and low bioactivity of bioresorbable polymers, such as poly(epsilon-caprolactone) (PCL) in bone tissue engineering applications. This research investigates the potential use of cellulose nanocrystals (CNCs) as multi-functional additives that enhance the mechanical properties and increase the biomineralization rate of PCL. To this end, an in vitro biomineralization study of both sulfuric acid hydrolyzed-CNCs (SH-CNCs) and surface oxidized-CNCs (SO-CNCs) has been performed in simulated body fluid in order to evaluate the bioactivity of the surface functional groups, sulfate and carboxyl groups, respectively. PCL nanocomposites were prepared with different SO-CNC contents and the chemical/physical properties of the nanocomposites were analyzed. 3D porous scaffolds with fully interconnected pores and well-controlled pore sizes were fabricated from the PCL nanocomposites with a 3D printer. The mechanical stability of the scaffolds were studied using creep test under dry and submersion conditions. Lastly, the biocompatibility of CNCs and 3D printed porous scaffolds were assessed in vitro. The carboxyl groups on the surface of SO-CNCs provided a significantly improved calcium ion binding ability which could play an important role in the biomineralization (bioactivity) by induction of mineral formation for bone tissue engineering applications. In addition, the mechanical properties of porous PCL nanocomposite scaffolds were pronouncedly reinforced by incorporation of SO-CNCs. Both the compressive modulus and creep resistance of the PCL scaffolds were enhanced either in dry or in submersion conditions at 37 degrees Celsius. Lastly, the biocompatibility study demonstrated that both the CNCs and material fabrication processes (e.g., PCL nanocomposites and 3D printing) were not toxic to the preosteoblasts (MC3T3 cells). Also, the SO-CNCs showed a positive effect on biomineralization of PCL scaffolds (i.e., accelerated calcium or mineral deposits on the surface of the scaffolds) during in vitro study. Overall, the SO-CNCs could play a critical role in the development of scaffold materials as a potential candidate for reinforcing nanofillers in bone tissue engineering applications.
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    Biopolymer Structure Analysis and Saccharification of Glycerol Thermal Processed Biomass
    Zhang, Wei (Virginia Tech, 2015-01-31)
    Glycerol thermal processing (GTP) is studied as a novel biomass pretreatment method in this research with the purposes to facilitate biopolymer fractionation and biomass saccharification. This approach is performed by treating sweet gum particles on polymer processing equipment at high temperatures and short times in the presence of anhydrous glycerol. Nine severity conditions are studied to assess the impact of time and temperature during the processing on biopolymer structure and conversion. The GTP pretreatment results in the disruption of cell wall networks by increasing the removal of side-chain sugars and lignin-carbohydrate linkages based on severity conditions. After pretreatment, 41% of the lignin and 68% of the xylan is recovered in a dry powdered form by subsequent extractions without additional catalysts, leaving a relatively pure cellulose fraction, 84% glucan, as found in chemical pulps. Lignin structural analysis indicated GTP processing resulted in extensive degradation of B-aryl ether bonds through the C-y elimination, followed by abundant phenolic hydroxyl liberation. At the same time, condensation occurred in the GTP lignin, providing relatively high molecular weight, near to that of the enzymatic mild acidolysis lignin. Better thermal stability was observed for this GTP lignin. In addition to lignin, xylan was successfully isolated as another polymer stream after GTP pretreatment. The recovered water insoluble xylan (WIX) was predominant alkali soluble fraction with a maximum purity of 84% and comparable molecular weight to xylan isolated from non-pretreated fibers. Additionally, the narrow molecular weight distribution of recovered WIX, was arisen from the pre-extraction of low molecular weight water-soluble xylan. Additionally, a 20-fold increase of the ultimate enzymatic saccharification for GTP pretreated biomass was observed even with significant amounts of lignin and xylan remaining on the non-extracted fiber. The shear and heat processing caused a disintegrated cell wall structure with formation of biomass debris and release of cellulose fibrils, enhancing surface area and most likely porosity. These structural changes were responsible for the improved biomass digestibility. Additionally, no significant inhibitory compounds for saccharification are produced during GTP processing, even at high temperatures. While lignin extraction did not promote improvement in hydrolysis rates, further xylan extraction greatly increases the initial enzymatic hydrolysis rate and final level of saccharification. The serial of studies fully demonstrate glycerol thermal processing as a novel pretreatment method to enhance biomass saccharification for biofuel production, as well as facilitate biopolymer fractionation. Moreover, the study shows the impact of thermally introduced structural changes to wood biopolymers when heated in anhydrous environments in the presence of hydrogen bonding solvent.
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    Blends of Biodegradable Thermoplastics With Lignin Esters
    Ghosh, Indrajit (Virginia Tech, 1998-04-22)
    Thermoplastic blends of several biodegradable polymers with lignin (L) and lignin esters were prepared by solvent casting and melt processing. Among the biodegradable thermoplastics were cellulose acetate butyrate (CAB), poly-hydroxybutyrate (PHB), poly-hydroxybutyrate-co-valerate (PHBV), and a starch-caprolactone blend (SCL). Lignin esters included acetate (LA), butyrate (LB), hexanoate (LH), and laurate (LL). Blend characteristics were analyzed in terms of thermal and mechanical properties. The results indicate widely different levels of interaction between two polymer constituents. Melt blended samples of CAB/LA and CAB/LB were compatible on a 15-30 nm scale when probed by dynamic mechanical thermal analysis, and the glass transition temperatures of the blends followed Fox equation, whereas those of CAB/LH and CAB/LL showed distinct broad transitions on the same scale. Melt blending produced well dispersed phases whereas large phase separation evolved out of solvent castings. Crystallinity and melting points of PHB and PHBV were affected by the incorporation of lignin component, revealing some interaction between the blend constituents. Blends of SCL with L and LB revealed significant effect on crystallinity and melting temperatures of poly-caprolactone component, revealing polymer-polymer interaction between SCL and lignin components. An increased degree of crystallinity was observed in the case of higher-Tg L compared to lower Tg LB. Improvememt in modulus (and in some cases strength also) was observed in almost all blends types due to the glassy reinforcing behavior of lignin.
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    Carboxymethylcellulose Acetate Butyrate Water-Dispersions as Renewable Wood Adhesives
    Paris, Jesse Loren (Virginia Tech, 2010-08-05)
    Two commercial carboxymethylcellulose acetate butyrate (CMCAB) polymers, high and low molecular weight (MW) forms, were analyzed in this study. High-solids water-borne dispersions of these polymers were studied as renewable wood adhesives. Neat polymer analyses revealed that the apart from MW, the CMCAB systems had different acid values, and that the high MW system was compromised with gel particle contaminants. Formulation of the polymer into water-dispersions was optimized for this study, and proved the "direct method", in which all formulation components were mixed at once in a sealed vessel, was the most efficient preparation technique. Applying this method, 4 high-solids water dispersions were prepared and evaluated with viscometry, differential scanning calorimetry, dynamic mechanical analysis, light and fluorescence microscopy, and mode I fracture testing. Thermal analyses showed that the polymer glass transition temperature significantly increased when bonded to wood. CMCAB dispersions produced fairly brittle adhesive-joints; however, it is believed toughness can likely be improved with further formulation optimization. Lastly, dispersion viscosity, film formation, adhesive penetration and joint-performance were all dependent on the formulation solvents, and moreover, these properties appeared to correlate with each other.
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    Catechyl-lignin tissues in Vanilla orchid and Candlenut: structure/property studies
    Ristanti, Eky Yenita (Virginia Tech, 2023-05-24)
    In 2012, a new type of lignin, catechyl (C)-lignin was found in the seed coat of vanilla orchid (Vanilla planifolia) and Melocactus cacti, and later in the nutshell of Aleurites moluccana (candlenut). This caffeyl alcohol homopolymer is the exclusive lignin in vanilla seed coat but separated in time and/or location with guaiacyl (G)-lignin in candlenut. Unlike conventional guaiacyl/syringyl (G/S-lignins) with alkyl-aryl ether linkages, intermonomer linkages in C-lignin are connected by benzodioxane linkages which are stiffer than alkyl-aryl ether linkages. C-lignin is unusually stable against acid-catalyzed cleavage. Tissues with C-lignin are expected to exhibit high glass transition temperature (Tg) compared to tissues with G/S/H-lignin. C-lignin also probably shows high crystallinity due to its highly linear-homopolymer structure. The ability of some seed coats/nutshells in angiosperms to synthesize a new type of lignin is another level of lignin evolution. However, the role of C-lignin related to the function of the seed coat is unclear while it exhibits different behaviors to the regular G/S/H-lignin. These points motivated us to conduct cell-wall structure/property studies in the context of plant evolution, using microscopy, X-ray diffraction (XRD) and dynamic mechanical analysis (DMA). Light and electron microscopes were used to identify cell's size and type of intact and macerated vanilla seed coat and candlenut shell. Vanilla seeds are tiny, sized approximately 300μm and the surface is covered with dark-colored seed coat. Candlenut is slightly smaller than walnut, with uneven, hard, dark brown shell covering the nut. Microscopy observations indicated that both seed coat and nutshell are dominated by highly lignified cells, known as sclereids. The types of sclereids in vanilla seed coat and candlenut shell are different; vanilla seed coat has ostoesclereid-type cells, while candlenut shell has macrosclereid-type cells. XRD was used to study tissue with C-lignin crystallinity by comparing diffractograms of vanilla seed coat and candlenut shell to Southern Yellow Pine wood diffractograms. The Southern Yellow Pine wood diffractogram corresponds to a typical native cellulose in higher plants, that is cellulose I allomorph. Diffractogram XRD analysis on vanilla seed coat and candlenut shell shows similarities to Southern Yellow Pine native cellulose, suggesting that cellulose is the contributor for crystallinity in seed coat and nutshell, and this also indicated that tissues with C-lignin is not crystalline. Crystallinities of vanilla seed coat and candlenut shell determined using peak deconvolution methods were about half of Southern Yellow Pine crystallinity. DMA was used to measure Tg in vanilla seed coat and candlenut shell. Measurements were conducted in solvent-submersion mode using organic plasticizers to reduce the Tg to non-damaging temperatures. DMA measurement of vanilla seed coat and candlenut shell is challenging due to specimen size and shape. Specimen preparation for DMA measurement included seed coat purification for vanilla and cutting/milling for candlenut shell followed by specimen saturation in plasticizers. Compressive-torsion DMA was used to allow tiny specimens gripping. Vanilla seed coats exhibited higher glass transition temperature compared to wood, while candlenut shells exhibited various Tgs depending on specimen type/size.
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    Cellulose fiber reinforced thermoplastic composites: Processing and Product Charateristics
    Taib, Razaina Mat (Virginia Tech, 1998-05-18)
    Steam exploded fibers from Yellow Poplar (Liriodendron tulipifera) wood were assessed in terms of (a) their impact on torque during melt processing of a thermoplastic cellulose ester (plasticized CAB); (b) their fiber incorporation and dispersion characteristics in a CAB-based composite by SEM and image analysis, respectively; and (c) their impact on the mechanical properties (under tension) of CAB-based composites having fiber contents of between 10 and 40% by weight. The fibers included water-washed steam exploded fibers (WEF), alkali-extracted fibers (AEF), acetylated fibers (AAEF), all from Yellow poplar (log Ro = 4.23), and oat fillers (COF) as control. The stepwise increase in cellulose content by extraction, and especially the (surface) modification by acetylation, contributed to increased torque during melt processing, and to improved interfacial adhesion as well as fiber dispersion. As compared to pure CAB, AAEF generated the highest increase in torque (+ 421%) followed by AEF (+ 260%) and WEF (+ 190%) at 40% fiber content by weight. AAEF was also found to enhance the tensile properties of the resulting composites. SEM studies of the tensile fracture surfaces indicated significant interfacial delamination and also pull - out of fibers when WEF, AEF, and COF were used to reinforce the CAB matrix. Composites with AAEF, by contrast, revealed fracture surfaces with reduced interfacial delamination and with significant fiber fracturing during failure. Image analysis was used to determine fiber dispersion within the resulting composites quantitatively. Significant improvement in fiber dispersion was achieved when the matrix was reinforced with acetylated fibers (AAEF). Fiber addition to the matrix resulted in loss of strain at break (- 80 to - 93%) and slight or significant increases in modulus (+ 47 to + 103%) depending on fiber type at 40% fiber content. Maximum stress declined for all fibers except AAEF at all fiber contents. AAEF-based composites revealed a decline in maximum stress when fiber content rose to 10%, and this reversed when fiber content increased beyond 10%. This increase in strength is consistent with the rule of mixtures that stipulates reinforcement of the matrix by fibers that are capable of transferring stresses across the fiber-matrix interface. All fibers suffered length decreases during melt processing.
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    Characterization of Laser Modified Surfaces for Wood Adhesion
    Dolan, Jeffrey Alan (Virginia Tech, 2014-07-01)
    The controlled degradation of wood surfaces with infrared light from a CO2 pulsed laser facilitated adhesion without the use of additional resins. Laser modification creates a surface phenomenon that physically and chemically alters the natural biopolymer organization of lignocellulosic materials in a way that promotes adhesion when hot pressed using typical industrial equipment. Laser optimization was determined through mechanical and microscopic observation. It was determined that a mild level of laser surface modification (scale of 30 W/mm2) resulted in the highest bond-line strength. The large spot size of the laser beam resulted in evenly modified surfaces. Surface analysis revealed that laser modification changed native wood morphology, hydrolyzed and vaporized hemicellulose, and enriched the surface with cellulose II and lignin. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR FTIR) was used to analyze the bulk of the laser material. This experiment revealed a change in the hydroxyl region related to hydrogen bonding conformations between wood polymers, mainly cellulose. X-ray photoelectron spectroscopy (XPS) provided an elemental composition of the top 5 nanometers of the surface, which resulted in increased carbon-carbon/carbon-hydrogen linkages and decreased oxygen containing bonds due to laser ablation. Static acid-base contact angle analysis was conducted using three probe liquids to find the Lewis acid, Lewis base, and dispersion components of the top nanometer of surface chemistry. Contact angle analysis revealed laser modified samples had a surface free energy that remained similar to the control wood sample. In addition, the dispersion component of the surface free energy increased due to laser ablation while acid-base components were reduced. Atomic force microscopy (AFM) visually displays a reduction in surface roughness due to the laser technique. An additional set of experiments like thermal gravimetric analysis, thermal pre and post treatments, and heated ATR FTIR and XPS support findings which require more investigation into this adhesion phenomenon.
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    Characterization of PF Resol/Isocyanate Hybrid Adhesives
    Riedlinger, Darren Andrew (Virginia Tech, 2008-02-01)
    Water-based resol phenol formaldehyde, PF, and organic polymeric methylenebis(phenylisocyanate), pMDI, are the two primary choices for the manufacture of exterior grade wood-based composites. This work addresses simple physical blends of pMDI dispersed in PF as a possible hybrid wood adhesive. Part one of this study examined the morphology of hybrid blends prepared using commercially available PF and pMDI. It was found that the blend components rapidly reacted such that the dispersed pMDI droplets became encased in a polymeric membrane. The phase separation created during liquid/liquid blending appeared to have been preserved in the cured, solid-state. However, substantial interdiffusion and copolymerization between blend components also appeared to have occurred according to measured cure rates, dynamic mechanical analysis, and atomic force microscopy. In the second part of this study a series of PF resins was synthesized employing the so-called "split-cook" method, and by using a range of formaldehyde/phenol and NaOH/phenol mole ratios. These neat PF resins were subjected to the following analyses: 1) steady-state flow viscometry, 2) free formaldehyde titration, 3) non-volatile solids determination, 4) size exclusion chromatography, 5) quantitative solution-state ¹³C nuclear magnetic resonance, NMR, 6) differential scanning calorimetry, 7) parallel-plate oscillatory cure rheology, and 8) dielectric spectroscopy. The neat PF analytical results were unremarkable with one exception; NMR revealed that the formaldehyde/phenol mole ratio in one resin substantially differed from the target mole ratio. The neat PF resins were subsequently used to prepare of series of PF/pMDI blends in a ratio of 75 parts PF solids to 25 parts pMDI solids. The resulting PF/pMDI blends were subjected to the following analyses: 1) differential scanning calorimetry, 2) parallel-plate oscillatory cure rheology, and 3) dielectric spectroscopy. Similar to what was inferred in part one of this study, both differential scanning calorimetry (DSC) and oscillation cure rheology demonstrated that cure of the PF continuous phase was substantially altered and accelerated by pMDI. However within actual wood bondlines, dielectric analysis detected little variation in cure speed between any of the formulations, both hybrid and neat PF. Furthermore, the modulated DSC curing experiments detected some latent reactivity in the hybrid system, both during initial isothermal curing and subsequent thermal scanning. The latent reactivity may suggest that a significant diffusion barrier existed between blend components, preventing complete reaction of hybrid blends even after thermal scanning up to 200 °C. Part three of this work examined the bonded wood mode-I fracture performance of hybrid resins as a function of the resol formaldehyde/phenol ratio and also the alkali content. A moderate increase in unweathered fracture toughness was observed for hybrid formulations relative to neat PF. Following accelerated weathering, the durability of the hybrid blends was promising: weathered hybrid toughness was equivalent to that of weathered neat PF. While the resol F/P ratio and alkali content both influenced hybrid fracture toughness, statistical modeling revealed interaction between these variables that complicated result interpretation: the influence of hybrid alkali content depended heavily on each formulation's specific F/P ratio, and vice versa.
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    Characterizing the Durability of PF and pMDI Adhesive Wood Composites Through Fracture Testing
    Scoville, Christopher R. (Virginia Tech, 2001-06-13)
    The increased use of wood composites in building materials results in a need for a better understanding of wood adhesion. The effects of water and temperature exposure on the durability of wood products were assessed using the double-cantilever beam (DCB) method of fracture testing. The relative durability of phenol-formaldehyde (PF) and isocyanate (pMDI) adhesives was compared using a 2-hour boil test and an environmental test. The feasibility of using oriented strandboard (OSB), oriented strand lumber (OSL) and parallel strand lumber (PSL) for the DCB fracture method was assessed. The fracture resistance of PF was reduced significantly by the aging exposures. The fracture resistance of pMDI did not decrease after the 2-hour boil test. The DCB fracture method was shown to be useful with a square-grooved machined specimen using OSB and OSL.
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    Chemical Modification of Alginates in Organic Media
    Pawar, Siddhesh Nitin (Virginia Tech, 2013-07-25)
    Alginates are (1and4) linked linear copolysaccharides composed of B-D-mannuronic acid (M) and its C-5 epimer, a-L-guluronic acid (G). Several strategies to synthesize organically modified alginate derivatives have been reported, but almost all chemistries are performed in either aqueous or aqueous-organic media. The ability to react alginates homogeneously in organic solvents would open up access to a wide range of new chemistries and derivatives. However, past attempts have been restricted by the absence of methods for alginate dissolution in organic media. We therefore report a strategy to solubilize tetrabutylammonium (TBA) salts of alginic acid in polar aprotic solvents containing tetrabutylammonium fluoride (TBAF). Acylation of TBA-alginate was performed in DMSO/TBAF to get products with DSacetyl up to ~ 1.0. We further report that by using appropriate solvent conditions, placement of acyl groups can be controlled to achieve either random or M-selective substitution. Alginate acetates synthesized in an M-selective fashion were used to study the ability of these derivatives to form Ca-crosslinked hydrogels. Detailed structure-property analyses were performed to identify acetylation reaction conditions and product properties that may be ideal for hydrogel formation. Furthermore, alginate esters were synthesized via modification of carboxylate groups on the backbone. These derivatives dissolved in polar aprotic solvents without the need to add TBAF. A proof of concept study showed their utility in the solubility enhancement of the poorly water soluble flavonoid naringenin.
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    Chemical modification of shaped hydrogels in non-aqueous medium
    (United States Patent and Trademark Office, 1996-06-25)
    A method is described for the chemical modification of pre-shaped hydrogels in non-aqueous medium. The conditions permit the reaction of highly expanded, porous hydrogel particles, such as spherical beads, using pseudo homogeneous reaction conditions in the absence of water. The method involves a three step procedure in which the porous gels are solvent exchanged int a water-free solvent (step 1) with minimal change in gel dimension and porosity; followed by reaction under non-aqueous condition (step 2); and followed by solvent exchange into water (step 3). Many different types of reactions requiring non-aqueous conditions may be carried out using these conditions. The method has particularly been demonstrated for crosslinking fluorinating beads, and for esterifying beads in a reaction involving multifunctional free carboxylic acids in the presence of dicyclohexylcarbodiimide (DCC).
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    Comparative Analysis of Inactivated Wood Surfaces
    Sernek, Milan (Virginia Tech, 2002-04-24)
    A wood surface, which is exposed to a high temperature condition, can experience inactivation. Surface inactivation results in reduced ability of an adhesive to properly wet, flow, penetrate, and cure. Thus, an inactivated wood surface does not bond well with adhesives. The changes in surface chemistry, wettability, and adhesion of inactivated wood surfaces, including heartwood of yellow-poplar (Liriodendron tulipifera) and southern pine (Pinus taeda), were studied. Wood samples were dried from the green moisture content condition in a convection oven at five different temperature levels ranging from 50 to 200 °C. The comparative characterization of the surface was done by X-ray photoelectron spectroscopy (XPS), sessile drop wettability, and fracture testing of adhesive bonds. The oxygen to carbon ratio (O/C) decreased and the C1/C2 ratio increased with drying temperature. The C1 component is related to carbon-carbon or carbon-hydrogen bonds, and the C2 component represents single carbon-oxygen bond. A low O/C ratio and a high C1/C2 ratio reflected a high concentration of non-polar wood components (extractives/VOCs) on the wood surface, which modified the wood surface from hydrophilic to more hydrophobic. Wettability was directly related to the O/C ratio and inversely related to the C1/C2 ratio. Contact angle decreased with time and increased with the temperature of exposure. Southern pine had a lower wettability than yellow-poplar, which was due to a greater concentration of non-polar hydrocarbon-type extractives and heat-generated volatiles on the surface. Solvent extraction prior to drying did not improved wettability, whereas, extraction after drying improved wettability. A contribution of extractives migration and VOCs generation played a significant role in the heat-induced inactivation process of southern pine. The maximum strain energy release rate (Gmax) showed that surface inactivation was insignificant for yellow-poplar when exposed to drying temperatures < 187°C. The southern pine was most susceptible to inactivation at drying temperatures > 156°C, particularly when bonded with phenol-formaldehyde (PF) adhesive. Chemical treatments improved the wettability of inactivated wood surfaces, but an improvement in adhesion was not evident for specimens bonded with polyvinyl-acetate (PVA) adhesive. NaOH surface treatment was most effective for improving adhesion of the PF adhesive bond.
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    Design, Processing, and Characterization of Nanocomposites
    Venkatraman, Priya (Virginia Tech, 2020-06-24)
    Structure and processing of polymer composites are essential for not only optimizing materials properties for high performance applications, but also making materials more environmentally sustainable. In the aerospace and automobile industries, the need for lightweight materials to increase fuel efficiency, while still boasting impressive mechanical properties drives innovation towards the manufacturing and use of more eco-friendly materials. In this dissertation, we concentrate on the processing and characterization of polymers reinforced with bio-based cellulose nanoparticles at an industrial scale. The successful incorporation of cellulose nanocrystals (CNCs) in polyamides, specifically polyamide 11 (PA 11) and polyamide 6 (PA 6), can result in a more sustainable material. However, unless polyamide nanocellulose composites can be produced at an industry scale, the use of traditional fillers like glass fibers will continue to be the industry standard despite their lack of being bio-based or providing comparable mechanical enhancement. Challenges of thermal stability, homogeneous dispersion, and moisture uptake have long served as the bottleneck to industrial-scale processing of these cellulose nanocomposites. To overcome these challenges, various industrially viable pre-mixing techniques, such as planetary ball milling, roller blade mixing, and master batching, are developed herein to fabricate these materials for melt processing while retaining the thermal and mechanical integrity of the nanocomposites. In this dissertation, successful high-temperature processing of these nanocellulose composites is shown with resultant nucleated materials showing up to approximately 75 % reinforcement of PA 6 and up to 180 % of PA 11. The efficacy of cellulose nanocrystals as a nucleating agent and the respective crystallization kinetics of the nanocomposite at process-relevant conditions were explored using fast scanning calorimetry. It was found that the PA 11 composite would need at least 0.5 s above 100 °C in order to crystallize via heterogeneous nucleation and that in the heterogeneous regime, the nucleated samples exhibited much quicker peak crystallization times. This successful development and optimization of these processing methods and parameters taps into the immense potential cellulose nanomaterials have in creating high performance, environmentally sustainable materials. Additionally, the use of these cellulose nanomaterials as templates for graphene oxide nanotubes is explored in this dissertation as well. Utilizing an inverting thermal degradation (iTD) method, the combustion kinetics of nanocellulose fibers were altered through surface hydrophobization and salt saturation to influence ignition propagation rate and ignition nucleating points. Samples were ignited at both slow and rapid rates, with flash burning of these nanofibers producing graphene oxide nanotubes through ignition of the bulk material and leaving behind the chemically altered surface structure of the nanofibers. Fiber aspect ratio, crystallinity, salt occlusion, ignition rate, and local oxygen availability proved influential in the resulting nanostructures, characterized through electron microscopy coupled with focused ion beam, solid state NMR, and XPS. Using this facile process and utilizing nanocellulose, the world's most abundant natural polymer, as the starting material, the work presented herein could have profound implications for the future production of environmentally sustainable carbon nanotubes. In the biomedical industry, new materials and processing techniques are required to match the increasing demand for personalized treatment. Furthermore, bone implants that promote osseointegration and long-term retention has instigated the search for materials to replace the traditional metal or ceramic implants that typically come with a high risk of immune rejection. In this dissertation, a novel processing technique to fabricate porous poly(ether ether ketone) (PEEK) is developed and studied for its potential application as a bone replacement material. A PEEK composite was created with the addition of nano hydroxyapatite (nHA), which has shown to improve the biocompatibility of the material. While hydroxyapatite-PEEK composites have previously been investigated for use in bone replacement, they have faced challenges with dispersion and ultimate mechanical tensile and compressive strength. The ethanol solvent-exchange based novel process presented herein produces a material with porous structuration that mimics the transition of cortical (compact) to cancellous (porous) bone while also having a homogeneous dispersion of hydroxyapatite throughout the material. With a density of 0.84 g cm−3 ± 0.18, this nanocomposite material exhibited compressive strength of up to 180 ± 15 MPa, which compares well to that of natural human cortical bone ranging from 100-230 MPa. This PEEK nanocomposite could provide a treatment alternative that displays a longer lifespan, lowers risk of immune rejection, and eliminates damage to surrounding tissue. The advancements in processing of nanocomposites presented in this work can greatly impact various industries from providing better medical care to reducing the carbon footprint of plastics.
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    Development and Characterization of Advanced Polymer Electrolyte for Energy Storage and Conversion Devices
    Wang, Ying (Virginia Tech, 2017-01-09)
    Among the myraid energy storage technologies, polymer electrolytes have been widely employed in diverse applications such as fuel cell membranes, battery separators, mechanical actuators, reverse-osmosis membranes and solar cells. The polymer electrolytes used for these applications usually require a combination of properties, including anisotropic orientation, tunable modulus, high ionic conductivity, light weight, high thermal stability and low cost. These critical properties have motivated researchers to find next-generation polymer electrolytes, for example ion gels. This dissertation aims to develop and characterize a new class of ion gel electrolytes based on ionic liquids and a rigid-rod polyelectrolyte. The rigid-rod polyelectrolyte poly (2,2'-disulfonyl-4,4'-benzidine terephthalamide) (PBDT) is a water-miscible system and forms a liquid crystal phase above a critical concentration. The diverse properties and broad applications of this rigid-rod polyelectrolyte may originate from the double helical conformation of PBDT molecular chains. We primarily develop an ionic liquid-based polymer gel electrolyte that possesses the following exceptional combination of properties: transport anisotropy up to 3.5×, high ionic conductivity (up to 8 mS cm⁻¹), widely tunable modulus (0.03 – 3 GPa) and high thermal stability (up to 300°C). This unique platform that combines ionic liquid and polyelectrolyte is essential to develop more advanced materials for broader applications. After we obtain the ion gels, we then mainly focus on modifying and then applying them in Li-metal batteries. As a next generation of Li batteries, the Li-metal battery offers higher energy capacity compared to the current Li-ion battery, thus satisfying our requirements in developing longer-lasting batteries for portable devices and even electric vehicles. However, Li dendrite growth on the Li metal anode has limited the pratical application of Li-metal batteries. This unexpected Li dendrite growth can be suppressed by developing polymer separators with high modulus (~ Gpa), while maintaining enough ionic conductivity (~ 1 mS/cm). Here, we describe an advanced solid-state electrolyte based on a sulfonated aramid rigid-rod polymer, an ionic liquid (IL), and a lithium salt, showing promise to make a breakthrough. This unique fabrication platform can be a milestone in discovering next-generation electrolyte materials.
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    Development of tension and compression creep models for wood using the time-temperature superposition principle
    Bond, Brian H. (Virginia Tech, 1993-06-03)
    To date there are no long-term creep models or practical methods to investigate the effect of creep on the safety and serviceability of modem wood structures and structural wood composites. Long-term creep models were developed for wood in tension and compression using the Time-Temperature Superposition Principle (TTSP). The principle states that the long-term response of a polymer at lower temperature is equivalent to the short-term response at a higher temperature. Accelerated creep tests were conducted in tension and compression using small clear specimens of Douglas-fir, southern pine and yellow-poplar. The specimens were tested at moisture contents of 6 %, 9 %, and 12 %, and at temperatures between 20°C and 80°C. The strain was measured using bonded strain gages. The individual creep compliance for each temperature was shifted along the log-time axis to obtain a "master" curve that describes the creep response of the specimens. All compliance curves also required vertical shifting. The experimental horizontal shift factors followed the Arrhenius formulation that describes the shift factor relation for polymers in the glassy region.