Mark T. Kortschot

Cellulose microfibrils: A novel method of preparation using high shear refining and cryocrushing

Abstract This paper describes a novel technique to produce cellulose microfibrils through mechanical methods. The technique involved a combination of severe shearing in a refiner, followed by high-impact crushing under liquid nitrogen. Fibers treated in this way were subsequently either freeze-dried or suspended in water. The fibers were characterized using SEM, TEM, AFM, and high-resolution optical microscopy. In the freeze-dried batch, 75% of the fibrils had diameters of 1 μm and below, whereas in the water dispersed batch, 89% of the fibrils had diameters in this range. The aspect ratio of the microfibrils ranged between 15 and 55 for the freeze-dried fibrils, and from 20 to 85 for the fibrils dispersed in water. These measurements suggest that the microfibrils have the potential to produce composites with high strength and stiffness for high-performance applications. The microfibrils in water were compounded with polylactic acid polymer to form a biocomposite. Laser confocal microscopy showed that the microfibrils were well dispersed in the polymer matrix.

REINFORCING POTENTIAL OF WOOD PULP DERIVED MICROFIBRES IN A PVA MATRIX

Abstract In this study, the reinforcing potential of cellulose “microfibres” obtained from bleached softwood kraft pulp was demonstrated in a matrix of polyvinyl alcohol (PVA). Microfibres are defined as fibres of cellulose of 0.1–1 μm in diameter, with a corresponding minimum length of 5–50 μm. Films cast with these microfibres in PVA showed a doubling of tensile strength and a 2.5-fold increase in stiffness with 5% microfibre loading. The theoretical stiffness of a microfibre was calculated as 69 GPa. The study also demonstrated that the strength of the composite was greater at 5% microfibre loading compared to 10% loading. Comparative studies with microcrystalline cellulose showed that the minimum aspect ratio of the reinforcing agent is more criticalthan its crystallinity in providing reinforcement in the composite.

Correction to the Fukuda-Kawata Young's modulus theory and the Fukuda-Chou strength theory for short fibre-reinforced composite materials

The theories for modulus and strength of short fibre-reinforced composite materials are based on the calculation of the force sustained by fibres crossing an arbitrary line perpendicular to the applied load, called the scan line, in a thin, rectangular specimen. The widely referenced Fukuda-Kawata modulus theory and the Fukuda-Chou strength theory are based on an apparently incorrect procedure for the calculation of the force sustained by the fibres crossing the scan line. The error is explained in detail by comparing the Fukuda-Kawata modulus theory and the Cox modulus theory. The magnitude of this error is calculated for specific cases.

The role of the resin fillet in the delamination of honeycomb sandwich structures

In this study, the mechanism of peel fracture (or core/skin delamination) in honeycomb-core sandwich panels was investigated, and the role of resin fillets on energy absorption was elucidated. Fillet deformation and fracture were shown to control the delamination resistance of sandwich panels, to a great extent. In fact the area fraction of fillets on the fracture surface proved to be very important. The delamination resistance data for a variety of panels, made with various materials, cores etc., collapsed to a single master curve when the peel energy was normalized by the delamination energy of the skin material, and this ratio was plotted against the residual area fraction of the resin remaining on the core side after the peel test. A detailed stick/slip mechanism for crack advance was proposed, and its existence was supported by both the load traces and by fractography.

Expanded Wood Fiber Polystyrene Composites: Processing–Structure–Mechanical Properties Relationships

In this study, processing–structure–mechanical properties relationships in expanded wood fiber polystyrene composites (EPSC) made with a physical blowing agent were investigated. A systematic investigation was performed based on a statistical experimental design. The samples were saturated with carbon dioxide at high pressure and ambient temperature and the saturated specimens were expanded at elevated temperatures. The relations between impact and tensile properties of EPSC and foaming process and structure were studied. Fiber content was found to be the most important parameter controlling impact strength and tensile modulus. The impact strength of EPSC was increased about three times when the fiber content increased to 20%. Using the Halpin–Tsai equation, a model was developed to relate tensile modulus to the density of EPSC.

Characterization of composite mesostructures and damage by de-ply radiography

Abstract In order to characterize composite mesostructures and their influence on mechanical properties, it is first necessary to detect and quantify these structures. In this paper, the various techniques for characterizing internal structure and damage progression are briefly discussed, and a new technique, de-ply radiography, is introduced. The new technique involves a combination of standard radiography and the well documented de-ply technique developed previously (Freeman, S. M., ASTM STP, 787 (1980) 50–64). The laminate is penetrated with a solution of zinc iodide and subsequently the resin is pyrolysed in a furnace. After pyrolysis, the individual laminae may be separated, and radiographs of these laminae provide a three dimensional map of the damage pattern. The resulting images are clear and easy to obtain and the technique can be integrated with standard radiography. De-ply radiography has some limitations, but also offers some advantages over other methods of resolving damage in three dimensions within a laminated composite.

A simplified determination of theJ-integral for paper

A novel, simplified, method for evaluating theJ-integral for paper sheets has been developed. The inaccuracy of the single-specimen method (proposed by Riceet al. [8]), when applied to the fracture toughness testing of paper, was found to result from an erroneous assumption about the relationship between the load and the displacement due to plasticity. A new method was developed theoretically; it is based on a modified assumption about this relationship. Using this method, theJ-integral can be evaluated with a single load-displacement curve together with a new parameter that must be separately evaluated using at least one additional specimen. Although the new method requires a minimum of two notched specimens, it provides more flexibility than previous modifications of the single-specimen method. The experimental study indicated that the present method yields values ofJc (the critical value of theJ-integral) which agree very closely with those obtained using the cumbersome multiple-specimen method, and is therefore more suitable for paper testing than other methods presently available.

Performance of long Canadian natural fibers as reinforcements in polymers

Fiber morphology has a significant effect on the mechanical properties of fiber/polymer composites. The performance of nine types of long wood fibers (initial aspect ratio of >40), two long agricultural fibers (initial aspect ratio of >40), and one short fiber wood flour (initial aspect ratio of = 5—10) are compared. The fibers were compounded in polypropylene in a Brabender mixer and subsequently injection molded. The longer natural fibers (both wood fibers and agricultural fibers) did not provide significant additional reinforcement when compared to the wood flour. The fibers were extracted from the final specimens and measured using a Fiber Quality Analyzer. They were found to be severely degraded by processing, while the wood flour morphology was only slightly modified. The degree of length degradation was found to be dependent on fiber strength.

Effect of mixing conditions on the morphology and performance of fiber-reinforced polyurethane foam:

Polyol derived from soybean oil was used as a natural source preparation to create an environmentally friendly polyurethane foam. In order to stiffen these foams, wood fibers were added to provide reinforcement at low cost, while preserving the environmental friendly nature of the material. Mixing is a crucial step in the manufacture of reinforced foams and determines the fiber distribution and cell structure and hence the performance of the material. In this study, the effect of stirring variables on the mechanical properties of polyurethane foams was investigated and foams made using hand mixing were compared to foams made with a mechanical stirrer. Cell morphologies of the reinforced foams were characterized using scanning electron microscopy, X-ray computed tomography, and 3D stereo microscopy. The mechanical performance of the reinforced foam was essentially independent of stirring time and mainly depended on the variation in stirring rate. For foams made with high fiber fractions using a mechanical stirrer, the fibers were not as effective in increasing the compressive strength and modulus as they were in foams prepared by hand stirring. This suggests that mechanical stirring causes damage to the fibers, particularly at high fiber content.

Investigating the Mechanical Response of Soy-Based Polyurethane Foams with Glass Fibers under Compression at various Rates

Polyurethane foams have very diverse mechanical properties making them suitable for a wide range of applications. Their dependence on petroleum-based constituents, however, has prompted research in the preparation and investigation of foams made from bio-based components, such as polyol derived from soybean oil. Short fiber reinforcement has been found to improve the compressive modulus and plateau stress of the foams. In this study, elastic glass fibers were used to reinforce viscoelastic polyurethane foams; the compressive behaviour of the resulting foam was observed at various strain rates. At all strain rates, the fibers reinforced the foams, improving the modulus and plateau stress when compared to the properties of neat foam. A coupling between the fiber content and strain rate-dependence was observed in the modulus and plateau stress of the foams. Lastly, despite the increase in strength and stiffness of the foams, addition of fibers did not reduce the energy absorption of the foams.