Interface characteristics and adhesion in cellulose/polypropylene composites

The work detailed in this thesis focuses on the cellulose/polypropylene system as a model system for the study of interface characteristics and adhesion in a wood fiber/thermoplastic system.; Surface modification of cellulose fiber was done using alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), and stearic acid. Mechanical properties of composites containing AKD or ASA surface-treated cellulose were generally poorer than composites with untreated cellulose, while composites with stearic acid-treated cellulose had slightly better properties. Characterization of cellulose surfaces using the Wilhelmy fiber wetting technique showed that these surface treatments decreased the acid/base character (i.e. hydrogen bonding) of the cellulose. This decrease corresponded well with a decrease in adhesion as measured by an internal bond test.; Optical micrographs of crystallizing polypropylene containing treated and/or untreated cellulose fibers reveal the appearance of a transcrystalline region around untreated fibers, while this region was not present when the cellulose was surface-treated. A mechanism for the appearance of transcrystallinity based on epitaxial nucleation was proposed.; Differential scanning calorimetry measurements verified the occurrence of polypropylene nucleation in the presence of untreated cellulose, while no nucleation was seen when surface-treated cellulose fibers were present in the melt. Data from these DSC measurements were used in a nonisothermal Avrami crystallization model to solve for the Avrami exponent, n. These results verified the effectiveness of surface treatments in reducing the natural nucleating ability of cellulose. Sample mixing and filler content also played a role in the crystallization process.; Ozone gas was also used as a surface modifier for cellulose and polypropylene. When ozonized polypropylene was mixed with untreated cellulose, mechanical properties peaked after 25 minutes of treatment time. When ozonized cellulose was mixed with untreated polypropylene, mechanical properties showed a peak at short treatment times. Mixing ozonized polypsopylene and ozonized cellulose produced composites with no additional enhancement in mechanical properties. Changes in polypropylene melt viscosity with ozonation appeared to be the dominant factor in determining mechanical properties for these composites.; Also, by controlling press variables, internal bond strength was increased by releasing trapped moisture during hot-pressing.