Doctor of Philosophy (PhD)


Renewable Natural Resources

Document Type



Creep behavior of natural fiber/polymer composites (NFPCs) was studied in response to the increasing application of this material as structural building products. Factors that influence creep behavior of the composites were investigated by analyzing creep curves of several different NFPC systems, which were designed for overall performance of the composites. Among different models, the 4-element Burgers type was mostly used for quantitative characterization of the creep curves to compare the properties of different composites. The parameters from the 4-element Burgers model were easily interpretable due to their physical meanings. Generalized Burgers models provided better fit by introducing extra Kelvin units, but they are more complicated. Indexed Burgers models performed better for creep curves within the primary stage in terms of both characterization and prediction. Creep prediction was attempted through two approaches: modeling and accelerated testing. Burgers models were proven unsuitable for long-term prediction if the creep test time was not long enough. Comparatively, the indexed Burgers and 2-parameter power law models performed better for prediction purposes. Accelerated creep tests were conducted at higher temperatures, and smooth curves were obtained based on the time-temperature superposition (TTS) principle. The accuracy of long-term prediction was unable to be evaluated due to the lack of long-term experimental data. Several factors were shown to affect the creep resistance of NFPCs. These include polymer matrix type, natural fiber loading, additives, temperature, and weathering treatment. PVC had higher creep resistance than HDPE, and HDPE showed better creep resistance than ultra-high molecular weight polyethylene (UHMWPE). Introducing engineering plastics to form microfibrils in HDPE matrix improved its creep performance. Certain recycled plastics had smaller creep deformation than the corresponding virgin resin. Adding natural fibers into polymer matrix greatly enhanced its creep resistance. The effect of a coupling agent on creep property of NFPCs was dependent on its modulus and coupling effect. UVA, an ultrafine titanium dioxide, slightly reduced the creep deformation of HDPE composites at a low loading level. Higher temperatures led to not only larger instantaneous deformations, but also to higher long-term creep rates. Weathering treatment also affected the creep properties of polymer and NFPCs.



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Committee Chair

Wu, Qinglin