Daniel F. Caulfield

Interphase effects on the mechanical and physical aspects of natural fiber composites

The interaction and adhesion between the fiber and matrix has a significant effect in determining the mechanical and physical behavior of fiber composites. The effect of the interface and interphase depends on several factors such as chemical composition (functional groups), molecular structure characteristics (branching, molecular weight distribution, cross-linking), and details of its physical state (above or below Tg, nature and degree of crystallinity). Natural fibers have complex and varying chemical structures that have uneven surface topographies. This creates difficulties in using single fiber composite testing to accurately evaluate the interfacial shear strengths, except for comparisons. A review of our interphase related research in natural fiber composites is presented. When using coupling agents it is well known that the tensile and flexural strengths increase dramatically in natural fiber reinforced composites. However, in the case of modulus, the results are more complex. For two ethylene-propylene impact copolymers, the uncoupled systems had much higher Youngs moduli than the coupled systems. The dynamic storage moduli of the uncoupled impact polymers were higher than the coupled composites at temperatures up to about 50°C. At higher temperatures the presence of the coupling agent resulted in higher storage moduli. Transcrystallinity may play an important role in this phenomenon. Creep and other long-term properties are also affected by the quality of the interphase, although the level of improvement decreases with an increase in the molecular weight of the matrix polymer. Coupling agents reduced the rate of water absorption and the moduli were less affected in blends with a higher concentration of coupling agents.

Crystallization Behavior of Polyamide-6 Microcellular Nanocomposites:

The crystallization behaviors of polyamide-6 (PA-6) and its nanocomposites undergoing the microcellular injection molding process are studied using Transmission Electron Microscopy (TEM), X-ray Diffractometer (XRD), Polarized Optical Microscopy (POM), and Differential Scanning Calorimetry (DSC). The relationships among the morphology, the mechanical property of the molded parts, and the crystallization behavior are investigated. With the addition of nanoclays in microcellular injection molded parts, the growth of the γ-form crystal is suppressed and the formation of γ-form crystals is promoted. Both nanoclay and dissolved gas have a big influence on PA-6 crystalline structures. The existence of nanoclay increases the initial crystallization rate. But with extra addition of nanoclays in the polymer matrix, the increase of crystallization rate is reduced. Microcellular injection molded nanocomposites with proper amount of nanoclays possess the maximum crystallization activation energy and produce a finer and denser microcell structure which leads to better mechanical properties.

Thermoplastic polyolefins as formaldehyde free binders in highly filled lignocellulosic panel boards: using glycerine as a processing aid in kenaf fiber polypropylene boards

A new technique was developed to make highly loaded (up to 95%) formaldehyde free natural fiber boards. The purpose of the paper is to report a broad study on 85% kenaf boards using linear thermoplastic polymers as the binder in preparing the boards to determine if these materials have potential in commercial applications by comparing them to other commercial materials. In these materials, linear thermoplastic polymer chains act as an adhesive and the product resembles a typical wood based panel (e.g., phenol formaldehyde fiber board). The process involved the use of small amount of glycerine in the fiber to enhance processibility in a thermo-kinetic mixer followed by hot pressing. In this paper, we report the properties of 85% by weight kenaf fiber boards using polypropylene as the adhesive. A maleated polypropylene was used to improve the adhesion and stress transfer between the adhesive and kenaf fiber. The addition of 2% by weight of glycerine based on the dry weight of kenaf fiber resulted in the best properties of the boards. Differential scanning calorimetric studies suggested that the glycerine had a little effect on the percent crystallinity of the matrix. Dynamic mechanical tests of the 85% boards showed some differences compared to conventional 60% by weight kenaf-PP composites. The 85% kenaf boards had a flexural strength of 75 MPa and a flexural modulus of 6.8 GPa with a specific gravity of 1.24. These properties are comparable to standard formaldehyde free high density hardboards with flexural strengths of 48.3 MPa and flexural modulus of 5.5 GPa, and a specific gravity of 1.28. This paper gives a broad overview of an initial study of these new materials.