Craig M. Clemons

Mechanical properties and morphology of impact modified polypropylene-wood flour composites

The mechanical properties and morphology of polypropylene/wood flour (PP/WF) composites with different impact modifiers and maleated polypropylene (MAPP) as a compatibilizer have been studied. Two different ethylene/propylene/diene terpolymers (EPDM) and one maleated styrene–ethylene/butylene–styrene triblock copolymer (SEBS–MA) have been used as impact modifiers in the PP/WF systems. All three elastomers increased the impact strength of the PP/WF composites but the addition of maleated EPDM and SEBS gave the greatest improvements in impact strength. Addition of MAPP did not affect the impact properties of the composites but had a positive effect on the composite unnotched impact strength when used together with elastomers. Tensile tests showed that MAPP had a negative effect on the elongation at break and a positive effect on tensile strength. The impact modifiers were found to decrease the stiffness of the composites. Scanning electron microscopy showed that maleated EPDM and SEBS had a stronger affinity for the wood surfaces than did the unmodified EPDM. The maleated elastomers are, therefore, expected to form a flexible interphase around the wood particles giving the composites better impact strength. MAPP further enhanced adhesion between WF and impact-modified PP systems. EPDM and EPDM–MA rubber domains were homogeneously dispersed in the PP matrix, the diameter of domains being between 0.1–1 μm.

Nanofibrillated cellulose (NFC) reinforced polyvinyl alcohol (PVOH) nanocomposites: properties, solubility of carbon dioxide, and foaming

Polyvinyl alcohol (PVOH) and its nanofibrillated cellulose (NFC) reinforced nanocomposites were produced and foamed and its properties—such as the dynamic mechanical properties, crystallization behavior, and solubility of carbon dioxide (CO2)—were evaluated. PVOH was mixed with an NFC fiber suspension in water followed by casting. Transmission electron microscopy (TEM) images, as well as the optical transparency of the films, revealed that the NFC fibers dispersed well in the resulting PVOH/NFC nanocomposites. Adding NFC increased the tensile modulus of the PVOH/NFC nanocomposites nearly threefold. Differential scanning calorimetry (DSC) analysis showed that the NFC served as a nucleating agent, promoting the early onset of crystallization. However, high NFC content also led to greater thermal degradation of the PVOH matrix. PVOH/NFC nanocomposites were sensitive to moisture content and dynamic mechanical analysis (DMA) tests showed that, at room temperature, the storage modulus increased with decreasing moisture content. The solubility of CO2 in the PVOH/NFC nanocomposites depended on their moisture content and decreased with the addition of NFC. Moreover, the desorption diffusivity increased as more NFC was added. Finally, the foaming behavior of the PVOH/NFC nanocomposites was studied using CO2 and/or water as the physical foaming agent(s) in a batch foaming process. Only samples with a high moisture content were able to foam with CO2. Furthermore, the PVOH/NFC nanocomposites exhibited finer and more anisotropic cell morphologies than the neat PVOH films. In the absence of moisture, no foaming was observed in the CO2-saturated neat PVOH or PVOH/NFC nanocomposite samples.

Fuel Pellets from Wheat Straw: The Effect of Lignin Glass Transition and Surface Waxes on Pelletizing Properties

The utilization of wheat straw as a renewable energy resource is limited due to its low bulk density. Pelletizing wheat straw into fuel pellets of high density increases its handling properties but is more challenging compared to pelletizing woody biomass. Straw has a lower lignin content and a high concentration of hydrophobic waxes on its outer surface that may limit the pellet strength. The present work studies the impact of the lignin glass transition on the pelletizing properties of wheat straw. Furthermore, the effect of surface waxes on the pelletizing process and pellet strength are investigated by comparing wheat straw before and after organic solvent extraction. The lignin glass transition temperature for wheat straw and extracted wheat straw is determined by dynamic mechanical thermal analysis. At a moisture content of 8%, transitions are identified at 53°C and 63°C, respectively. Pellets are pressed from wheat straw and straw where the waxes have been extracted from. Two pelletizing temperatures were chosen—one below and one above the glass transition temperature of lignin. The pellets compression strength, density, and fracture surface were compared to each other. Pellets pressed at 30°C have a lower density and compression strength and a tendency to expand in length after the pelletizing process compared to pellets pressed at 100°C. At low temperatures, surface extractives have a lubricating effect and reduce the friction in the press channel of a pellet mill while no such effect is observed at elevated temperatures. Fuel pellets made from extracted wheat straw have a slightly higher compression strength which might be explained by a better interparticle adhesion in the absence of hydrophobic surface waxes.

Instrumented impact testing of kenaf fiber reinforced polypropylene composites: effects of temperature and composition

An instrumented Izod test was used to investigate the effects of fiber content, coupling agent, and temperature on the impact performance of kenaf fiber reinforced polypropylene (PP). Composites containing 0—60% (by weight) kenaf fiber and 0 or 2% maleated polypropylene (MAPP) and PP/wood flour composites were tested at room temperature and between -50°C and +50°C. At room temperature, kenaf greatly reduced energy to maximum load (EML) in reversed notch tests but had little effect in notched tests. MAPP improved all test values. At —25°C, PP specimens changed from ductile to brittle. Kenaf composites containing MAPP consistently yielded higher EML values than did both unfilled PP specimens and wood flour composites in notched impact tests, over the temperature range investigated. The EML values for kenaf composites were about half those for unfilled PP specimens in reversed notch tests at room temperature, but performance was similar at low temperatures.

Spatially Resolved Characterization of Cellulose Nanocrystal- Polypropylene Composite by Confocal Raman Microscopy

Raman spectroscopy was used to analyze cellulose nanocrystal (CNC)–polypropylene (PP) composites and to investigate the spatial distribution of CNCs in extruded composite filaments. Three composites were made from two forms of nanocellulose (CNCs from wood pulp and the nano-scale fraction of microcrystalline cellulose) and two of the three composites investigated used maleated PP as a coupling agent. Raman maps, based on cellulose and PP bands at 1098 and 1460 cm−1, respectively, obtained at 1 μm spatial resolution showed that the CNCs were aggregated to various degrees in the PP matrix. Of the three composites analyzed, two showed clear existence of phase-separated regions: Raman images with strong PP and absent/weak cellulose or vice versa. For the third composite, the situation was slightly improved but a clear transition interface between the PP-abundant and CNC-abundant regions was observed, indicating that the CNC remained poorly dispersed. The spectroscopic approach to investigating spatial distribution of the composite components was helpful in evaluating CNC dispersion in the composite at the microscopic level, which helped explain the relatively modest reinforcement of PP by the CNCs.

Dynamic fracture toughness of cellulose-fiber-reinforced polypropylene : Preliminary investigation of microstructural effects

In this study, the microstructure of injection-molded polypropylene reinforced with cellulose fiber was investigated. Scanning electron microscopy of the fracture surfaces and X-ray diffraction were used to investigate fiber orientation. The polypropylene matrix was removed by solvent extraction, and the lengths of the residual fibers were optically determined. Fiber lengths were reduced by one-half when compounded in a high-intensity thermokinetic mixer and then injection molded. At low fiber contents, there is little fiber orientation; at high fiber contents, a layered structure arises. To better understand mechanisms of fracture under impact loading, dynamic fracture analysis was performed based on linear elastic fracture mechanics. Dynamic critical energy release rates and dynamic critical stress intensity factors were deduced from instrumented Charpy impact test measurements. Dynamic fracture toughness increased with cellulose content and with orientation of fibers perpendicular to the crack direction. A preliminary evaluation of a simple model relating the microstructure to the dynamic fracture toughness shows promise, but further work is needed to assess its validity.

Improvements in processing characteristics and engineering properties of wood flour-filled high density polyethylene composite sheeting in the presence of hollow glass microspheres

Hollow glass microspheres were introduced into wood flour/high density polyethylene composites by melt compounding in a twin-screw extruder. The prepared composites were subsequently converted to extruded profiles in order to obtain composite sheeting. The presence of hollow glass microspheres highly reduced the density of the extruded sheets down to 0.91 g/cc, while improving its flexural modulus. The presence of hollow glass spheres further improved the visual appearance by eliminating warpage. Thermal conductivity of the sheets was reduced down to 0.25 W/mK without significantly changing the melt viscosity. The morphological analysis indicated a satisfactory state of dispersion of hollow glass microspheres in the sheeting. The presence of hollow glass microspheres resulted in sharper contours of the extruded profiles and improved nailability and screwability.

Use of saltcedar and Utah juniper as fillers in wood-plastic composites.

Invasive and small-diameter species have become more prevalent, creating numerous environmental and ecological problems. One potential method to control and eliminate invasive species and thereby promote natural rangeland restoration is developing new, value-added uses for them. Saltcedar (Tamarisk ramosissima) and Utah juniper (Juniperus osteosperma) were investigated for use as fillers in wood–plastic composites (WPCs). The chemical composition and thermal stability of wood flours from both invasive species were compared with those of commercial pine wood flour. The wood flours were compounded with plastic and additives, and the viscosities of the composite melts containing the different species were compared. Composites produced from the compounded material by profile extrusion and injection molding were evaluated for mechanical performance, appearance, and weatherability. Saltcedar wood flour had the most minerals and water soluble extractives, which resulted in the lowest thermal stability and the lowest melt viscosity when compounded with high-density polyethylene. Injection-molded WPCs made from saltcedar or juniper were both considerably darker than those made with pine but performed similarly in accelerated weathering tests. Their mechanical properties were generally lower than those of the composites made from pine, but appropriate application selection and proper design could help compensate. Extruded WPCs were successfully made with each of the species. Producing WPCs from these composites appears technically feasible, although continued formulation development and durability evaluation are needed so that informed decisions regarding applications can be made. Economically feasible applications that use the advantageous properties of these species and that can tolerate or address the less desirable ones need to be identified and demonstrated.

Characterization and Processing of Nanocellulose Thermosetting Composites

Fiber-reinforced polymer composites have gained popularity through their advantages over conventional metallic materials. Most polymer composites are traditionally made with reinforcing fibers such as carbon or glass. However, there has been recent interest in sourcing these reinforcing fibers from renewable, natural resources. Nanocellulose-based reinforcements constitute a new class of these naturally sourced reinforcements. Some unique behavior of nanocellulose creates both opportunities and challenges. This chapter reviews the progress and some of the remaining issues related to the materials, processing, and performance of nanocellulose reinforced thermosetting composites.