Lawrence T. Drzal

Sustainable Bio-Composites from Renewable Resources: Opportunities and Challenges in the Green Materials World

Sustainability, industrial ecology, eco-efficiency, and green chemistry are guiding the development of the next generation of materials, products, and processes. Biodegradable plastics and bio-based polymer products based on annually renewable agricultural and biomass feedstock can form the basis for a portfolio of sustainable, eco-efficient products that can compete and capture markets currently dominated by products based exclusively on petroleum feedstock. Natural/Biofiber composites (Bio-Composites) are emerging as a viable alternative to glass fiber reinforced composites especially in automotive and building product applications. The combination of biofibers such as kenaf, hemp, flax, jute, henequen, pineapple leaf fiber, and sisal with polymer matrices from both nonrenewable and renewable resources to produce composite materials that are competitive with synthetic composites requires special attention, i.e., biofiber–matrix interface and novel processing. Natural fiber–reinforced polypropylene composites have attained commercial attraction in automotive industries. Natural fiber—polypropylene or natural fiber—polyester composites are not sufficiently eco-friendly because of the petroleum-based source and the nonbiodegradable nature of the polymer matrix. Using natural fibers with polymers based on renewable resources will allow many environmental issues to be solved. By embedding biofibers with renewable resource–based biopolymers such as cellulosic plastics; polylactides; starch plastics; polyhydroxyalkanoates (bacterial polyesters); and soy-based plastics, the so-called green bio-composites are continuously being developed.

Surface modifications of natural fibers and performance of the resulting biocomposites: An overview

A review of biocomposites highlighting recent studies and developments in natural fibers, bio-polymers, and various surface modifications of natural fibers to improve fiber-matrix adhesion is presented. One of the most important factors which determine the final performance of the composite materials is the quality of the fiber-matrix interface. A sufficient degree of adhesion between the surface of hydrophilic ligno-cellulosic natural fibers and the polymer matrix resin is usually desired to achieve optimum performance of the biocomposite. Dewaxing, alkali treatment, isocyanate treatment, peroxide treatment, vinyl grafting, bleaching, acetylation, and treatment with coupling agents are useful ways to improve fiber-matrix adhesion in natural fiber composites. Two major areas of biocomposites will be discussed in this article. One is the most predominant biocomposite currently being commercialized for semi-structural use in the durable goods industries, e.g. auto-industries, i.e. natural fiber-polypropylene composites. The second type is the biocomposites from natural fibers and biodegradable plastics. Two major classes of biodegradable plastics are available, one being derived from renewable resources and the second type being synthesized in the laboratory from petrochemical sources which can also be used as matrix materials to make value-added biocomposites.

Adhesion of Graphite Fibers to Epoxy Matrices: I. The Role of Fiber Surface Treatment

Abstract Adhesion between graphite fibers and epoxy matrices is a necessary and sometimes controlling factor in achieving optimum performance. Manufacturers′ proprietary fiber surface treatments promote adhesion without providing a basic understanding of the fiber surface properties altered through their use. This study has combined fiber surface chemistry, morphology, interfacial strength measurements and fracture characterization in order to elucidate the role of surface treatments. The results of this investigation lead to the conclusion that surface treatments designed to promote adhesion to epoxy matrix materials operate through a two-part mechanism. First, the treatments remove a weak outer fiber layer initially present on the fiber. Second surface chemical groups are added which increase the interaction with the matrix. Increases in fiber surface area are not an important factor in promoting fiber-matrix adhesion. In some cases the upper limit to fiber-matrix interfacial shear strength is the intri...

Comparison of methods for the measurement of fibre/matrix adhesion in composites

Abstract This paper reviews the most common state-of-the-art techniques—micro-bond, single-fibre fragmentation and microdebond/micro-indentation—for measuring fibre/matrix adhesion in composite materials. Following a discussion of the background for each technique and theoretical analysis, the experimental apparatus and procedure are described. Results of finite element and photoelastic analyses are presented to identify the state of stress that is induced in each specimen model of the three techniques. An objective comparison between the three single-fibre techniques to measure interfacial shear strength, and between three composite laminate techniques ([±45]s tensile, losipescu and short-beam shear), is made using AS-4 and AU-4 carbon fibres in an Epon 828 epoxy resin system as the experimental material. Finally, the advantages and limitations of each technique are discussed.

Adhesion of Graphite Fibers to Epoxy Matrices: II. The Effect of Fiber Finish

Abstract Reinforcing fibers are available from various manufacturers with matrix compatible “finishes” applied to them. Usually these finishes or coatings are 100–200 nm thick resin layers applied after surface treatment. Their function has been hypothesized as being to enhance adhesion through either protecting the fiber from handling damage, protecting the fiber surface reactivity, or improving fiber wettability. This study of finished and unfinished graphite fibers concludes that the mechanism by which an epoxy compatible finish operates is different from what has been hypothesized to date. The finish layer creates a brittle interphase layer between the fiber and matrix which increases the interfacial shear strength but at the expense of changing the failure mode from interfacial to matrix.

Nanometal-decorated exfoliated graphite nanoplatelet based glucose biosensors with high sensitivity and fast response.

We report the novel fabrication of a highly sensitive, selective, fast responding, and affordable amperometric glucose biosensor using exfoliated graphite nanoplatelets (xGnPs) decorated with Pt and Pd nanoparticles. Nafion was used to solubilize metal-decorated graphite nanoplatelets, and a simple cast method with high content organic solvent (85 wt %) was used to prepare the biosensors. The addition of precious metal nanoparticles such as platinum (Pt) and palladium (Pd) to xGnP increased the electroactive area of the electrode and substantially decreased the overpotential in the detection of hydrogen peroxide. The Pt-xGnP glucose biosensor had a sensitivity of 61.5+/-0.6 microA/(mM x cm(2)) and gave a linear response up to 20 mM. The response time and detection limit (S/N=3) were determined to be 2 s and 1 microM, respectively. Therefore, this novel glucose biosensor based on the Pt nanoparticle coated xGnP is among the best reported to date in both sensing performance and production cost. In addition, the effects of metal nanoparticle loading and the particle size on the biosensor performance were systematically investigated.

Fibre-matrix adhesion and its relationship to composite mechanical properties

Two major areas of enquiry exist in the field of fibre-matrix adhesion in composite materials. One is the fundamental role that fibre-matrix adhesion plays on composite mechanical properties. The other is what is the “best” method used to measure fibre-matrix adhesion in composite materials. Results of an attempt to provide an experimental foundation for both areas are reported here. A well-characterized experimental system consisting of an epoxy matrix and carbon fibres was selected in which only the fibre surface chemistry was altered to produce three different degrees of adhesion. Embedded single-fibre fragmentation tests were conducted to quantify the level of fibre-matrix adhesion. Observation of the events occurring at the fibre breaks led to the documentation of three distinct failure modes coincident with the three levels of adhesion. The lowest level produced a frictional debonding, the intermediate level produced interfacial crack growth and the highest level produced radial matrix fracture. High fibre volume fraction composites made from the same material were tested for on- and off-axis, as well as fracture, properties. Results indicate that composite results can be explained if both differences in adhesion and failure mode are considered. It will be further demonstrated that fibre-matrix adhesion is an “optimum” condition which has to be selected for the stress state that the interface will experience. The embedded single-fibre fragmentation test is both a valuable measurement tool for quantifying fibre-matrix adhesion as well as the one method which provides fundamental information about the failure mode necessary for understanding the role of adhesion on composite mechanical properties.

A round-robin programme on interfacial test methods

Abstract A round-robin programme has been undertaken to assess the compatibility in the micromechanical techniques used to evaluate the interfacial shear strength of the fibre/matrix bond in composite materials. The tests selected for evaluation were the single-fibre pull-out test, the microdebond test, the fragmentation test and the micro-indentation test. Twelve laboratories were invited to participate in this programme. Each laboratory was supplied with Caurtaulds XA fibre in the untreated condition and with a standard surface treatment, and a quantity of epoxy resin, hardener and catalyst, all from the same batch. Some laboratories were supplied with composite bars made with the same materials. A common cure cycle was chosen for sample preparation. Each laboratory conducted the tests to its own procedures. The results showed that the scatter within each laboratory was acceptable but the scatter between laboratories for a particular test was high. The results are discussed and possible explanations are presented for these observations. The indications are that the fundamental procedures used in each laboratory are sound. The results also suggest that there is great potential for achieving standard procedures and reducing the inter-laboratory scatter. A further round-robin programme is proposed to generate test protocols.

Fiber-Matrix Adhesion and Its Effect on Composite Mechanical Properties: II. Longitudinal (0°) and Transverse (90°) Tensile and Flexure Behavior of Graphite/Epoxy Composites:

An optimum level of interfacial bond strength between reinforcing fiber and a polymeric matrix in which it is placed is essential for acceptable composite mechanical properties and performance. The interfacial bond strength can be optimized only when the relationship between the level of fiber-matrix adhesion and the mechanical and fracture behavior of composites is clearly understood. This study establishes the relationship between the fiber-matrix interfacial shear strength and 0° and 90° tensile and flexure properties of graphite/epoxy composites. A well defined and characterized graphite fiber/epoxy system was chosen in which the level of adhesion between fiber and matrix was changed by using the same graphite fibers through the use of surface treatment and finish. The level of adhesion between the fiber and matrix associated with these changes resulted in an increase of fiber-matrix interfacial shear strength (ISS) by over a factor of two while the fiber and matrix properties remained unchanged. The experimental results demonstrated that the fiber surface modification did not have much effect on the tensile and flexural moduli and on the fiber dominated properties. However, the strengths and maximum strains that are governed by the matrix and interface properties were highly sensitive to the fiber surface modification. In addition, the major failure modes were also found to be affected by the fiber-matrix interfacial shear strength.

Fiber-Matrix Adhesion and Its Effect on Composite Mechanical Properties: IV. Mode I and Mode II Fracture Toughness of Graphite/Epoxy Composites

To optimize the level of fiber-matrix adhesion an understanding of the relationship between fiber-matrix interfacial bond strength and the mechanical and frac ture behavior of composites is essential. This study establishes the relationship between fiber-matrix interfacial shear strength (ISS) and interlaminar fracture toughness (both Mode I and Mode II) and failure modes for graphite/epoxy composites. A well defined and characterized graphite fiber/epoxy system was chosen in which the level of adhesion be tween fiber and matrix was changed by using the same graphite fibers with different sur face treatments. These surface treatments changed the level of adhesion between the fiber and matrix thus resulting in an increase of the fiber-matrix ISS by over a factor of two while the fiber and matrix properties remained unchanged. The Mode I and Mode II tests were conducted by the double cantilever beam (DCB) and end-notch flexure (ENF) tests methods, respectively. The Mode I fracture toughness (GIC ) of composites having low fiber-matrix ISS could not be determined from the DCB test because of extensive fiber bridging and crack meandering. For the composites having higher values of the ISS, the GIC increased with the ISS. The experimental results demonstrated that there is a strong dependency of Mode II fracture toughness (GIIC ) on fiber-matrix adhesion. Increased fiber-matrix adhesion in one group of composites significantly improved the GIIC , but the presence of brittle interphase around graphite fibers in another group of composites tended to cancel part of the improvement resulting from increased adhesion. Based on the major failure modes occurring during the Mode I and Mode II loading conditions, a causal link age between fiber-matrix adhesion and interlaminar fracture behavior of graphite/epoxy composites is established.