Thesis (Ph. D.)--University of Washington, 1999. / Vita. Includes bibliographical references (leaves 87-93).

Thesis (MScEng (Mechanical and Mechatronic Engineering)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: In the Greater Gariep agricultural area adjacent to the Orange River between Prieska and the Vanderkloof dam alone an estimated 311 000 ton/yr of maize and wheat straw is available. These agricultural residues have an energy equivalent of 196 000 ton of coal per year and should be utilised as a renewable energy resource. A technical and financial evaluation on the collection and transport of agricultural residue showed that the Hopetown area has the highest concentration of agricultural residue in the Greater Gariep agricultural area with approximately 68 000 ton/yr that is spread out over 76 kmª. Briquetting, combustion, pyrolysis and gasification were identified as the technologies with the highest potential to convert agricultural residue into a higher grade energy product in this area. The expected overall energy conversion efficiency for a plant capacity between 5 000 to 100 000 ton/yr is 98.9%, 10-25%, 25-30% and 28-36% for the briquetting, combustion, pyrolysis and gasification plants respectively. A financial evaluation based on the internal rate of return and the net present value of investment showed that the briquetting plant is financially feasible and the most profitable for capacities between 25 000 and 60 000 ton/yr while the pyrolysis plant was financially feasible and the most profitable technology for capacities greater than 60 000 ton/yr. A sensitivity and risk analysis done on the proposed briquetting and pyrolysis plants to evaluate the impact of market fluctuations on the profitability of the power plants exposed the briquetting plant as a very high risk investment, mainly because of the sensitivity to the selling price of fuel briquettes and the high maintenance cost associated with the briquetting equipment. Although the proposed pyrolysis plant is sensitive to variation in the electricity price, the risks associated with the market conditions for the pyrolysis plant is very low and an internal rate of return of 15% is still projected at the minimum expected electricity price. From the study it is clear that the utilisation of agricultural residue available in the Greater Gariep agricultural area is technically and financially viable. / AFRIKAANSE OPSOMMING: In die Groter Gariep landbougebied langs die Oranjerivier, tussen Prieska en die Van Der Kloof Dam is daar jaarliks ’n beraamde 311 000 ton mielie- en koringstrooi beskikbaar. Hierdie landbou-reste het die energie-ekwivalent van 196 000 ton steenkool per jaar en behoort as hernubare energiebron benut te word. ’n Tegniese en finansiële evaluasie van die versamel en vervoer van landbou-reste het getoon dat die Hopetown-area die hoogste konsentrasie landbou-reste in die Groter Gariep landbougebied het met ongeveer 68 000 ton/jaar wat versprei is oor 76 kmª. Brikettering, verbranding, pirolise en vergassing is geïdentifiseer as die tegnologieë met die hoogste potensiaal om landbou-reste te omskep in ’n hoër graad energieproduk vir hierdie gebied. Die verwagte totale energie-omsettingseffektiwiteit vir ’n aanlegkapasiteit van tussen 5 000 tot 10 000 ton/jaar is onderskeidelik 98.9%, 10-25%, 25-30% en 28-36% vir die brikettering, verbranding, pirolise en vergassingsaanlegte. ’n Finansiële evaluasie gebaseer op die opbrengs op aanvangskoste en die netto huidige waarde van die belegging het getoon dat die briketteringsaanleg finansieel lewensvatbaar is en die winsgewendste is vir ’n aanlegkapasiteit tussen 25 000 en 60 000 ton/jaar terwyl die pirolise-aanleg finansieel lewensvatbaar is en die winsgewendste tegnologie is vir kapasiteite van groter as 60 000 ton/jaar. ’n Sensitiwiteits- en risiko-analise is op die voorgestelde brikettings- en pirolise-aanlegte gedoen om die impak van markskommelings op die winsgewendheid van die aanlegte te evalueer. Die resultate het getoon dat die briketteringsaanleg ’n baie hoë-risiko belegging is as gevolg van die sensitiwiteit op die verkoopprys van brikette en die hoë onderhoudskoste van briketteringstoerusting. Alhoewel die voorgenome pirolise-aanleg sensitief is vir skommelings in die elektrisiteitsprys, is die risiko’s wat met die marktoestande vir die pirolise-aanleg gepaardgaan, baie laag en ’n opbrengs op aanvangskoste van 15% word steeds voorspel teen die minimum verwagte verkoopsprys van elektrisiteit. Vanuit die studie blyk dit duidelik dat die gebruik van landbou-reste wat beskikbaar is in die Groter Gariep landbougebied, tegnies en finansieel lewensvatbaar is as hernubare energiebron. / Sponsored by the Centre for Renewable and Sustainable Energy Studies

Thesis (M.S.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 156-165). Also available on the World Wide Web.

This thesis reports research work in the development of biocomposites based on whole utilisation of renewable wheat straw for industrial applications. The concept of “whole utilisation” is based on a previous work on a novel twin-screw extrusion technology for processing of feedstock in wheat straw reinforced bio-composites. It demonstrated that straw raw material could be restructurised into a feedstock with cellulose fibre finely dispersed in the non-cellulose matrix, which can be utilised as a bonding phase without having to be removed as in conventional processes to extract the cellulose. The whole straw can thus be utilised to avoid waste of materials and the negative impacts to environment associated with the extraction process. Raw wheat straw in this research was prepared in three ways: size reduction through mechanical milling, pre-treatment by aqueous NaOH solution and deep preparation with aqueous NaOH solution soaking followed by extrusion fractionation. Prepared wheat straws were processed into varieties of forms according to the applications. They were hot-compressed into self-reinforced composite with good flow ability and also processed through extrusion and compression moulding to compound with other biopolymers as good filler. The relationships of processing parameter and property, as well as formulation and property were established for each form of product, which provides a key understanding of the whole development circle of an end product. Through this research, scientific and technical problems has been addressed in materials formulation/processing, product design/manufacturing, enhancement of functionality/ performance as well as economical/environmental assessment so as to develop a series of cost-effective bio-composites and products, which satisfy diverse technical and environmental performance requirements in the industrial sectors across packaging, horticulture, building/construction and shooting sports.

Thesis (MSc)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: The microbial degradation of the lignin carbohydrate complex of plant material is only partially understood. Lignin carbohydrate complex was extracted from wheat straw and subsequently analysed. An adjustment to the standard protocol was required to increase the amount of lignin carbohydrate complex extracted from wheat straw. Characterization of the lignin carbohydrate complex after trifluoacetic acid hydrolysis was done by capillary electrophoresis. HPLC proved ineffective, as baseline separation could not be achieved. Characterization of the lignin carbohydrate complex revealed that a large portion (68 %) consisted of carbohydrate and lignin (20 %). Capillary electrophoresis of the trifluoroacetic acid hydrolysates of the lignin carbohydrate complex revealed that the carbohydrates consisted of mannose, xylose, arabinose, galactose, glucose and rhamnose. The major monosaccharide present in the lignin carbohydrate complex was mannose which made up 34 % of the total carbohydrate composition. Ferulic and p-coumaric acid were present in the lignin carbohydrate complex, but in concentrations less than 1 % of the lignin carbohydrate complex. The lignin carbohydrate complex of wheat straw probably had a heterogenous structure consisting of a variety of molecules, as molecular weight determination could not be accurately determined. An estimated molecular weight of 5.9 kOa was determined. Ten fungal strains (Aspergillus niger, Aureobasidium pul/u/ans, Bjerkandera adusta, Corio/us versicolor, Lenzitus betu/ina, Phanerochaete chrysosporium, Pycnoporus coccineus, Pycnoporus sanguineus 294, Pycnoporus sanguineus K5-2-3 and Trichoderma reeseï; were evaluated for growth on the lignin carbohydrate complex. All strains except B. adusta showed growth after 5 days with A. niger, A. pul/u/ans, C. versicolor, P. chrysosoporium and T. reesei showing the best growth on the lignin carbohydrate complex. The culture fluid revealed a number of proteins secreted by these organisms. The protein determination was confirmed by SOS-PAGE which revealed an array of proteins ranging from 8 kOa to 180 kOA. Prominent bands between 26 kOa and 80 kOa could be observed in the culture fluid of A. niger, A. pul/ulans and T. reesei, but not in C. versicolor. Activity studies on the culture fluid of these four strains revealed activity for xylanase, xylosidase, arabinofuranosidase, ferulic acid esterase and laccase, with vast differences between the activities of the various fungi. The enzymes of these fungal strains were also evaluated for their ability to degrade xylan and sugar cane bagasse using capillary electrophoresis. It appeared that all the organisms produced enzymes to degrade birchwood xylan. However, the electropherograms revealed that the degradation patterns of birchwood xylan differed among these organisms over the same time interval, as xylotetraose, xylotriose, xylobiose and xylose were released in various concentrations. The electropherograms obtained from the enzyme hydrolysates of sugar cane bagasse, pointed to the substrate being inaccessible. Electropherograms of the culture fluid of A. niger, A. pul/ulans, C. versicolor and T. reesei, when incubated on the lignin carbohydrate complex indicated similar peaks to those obtained and identified in the trifluoroacetic acid hydrolysates. However, the electropherograms of the culture fluid of these organisms revealed additional smaller peaks which could not be identified. The electropherograms of the culture fluid of the various organisms also indicated a complete release of some sugars, using the trifluoacetic acid hydrolysate of the lignin carbohydrate complex as a control for the amount of sugars present. HPLC analyses revealed that after 72 h, no apparent degradation of the lignin carbohydrate complex took place as peak height and areas appeared to be similar. These peaks could however not be identified due to a lack of standards as well as baseline separation which could not be achieved. / AFRIKAANSE OPSOMMING: Tans word die mikrobiese afbraak van die lignienkoolhidraatkompleks van plant materiaal slegs gedeeltelik verstaan. Lignienkoolhidraatkompleks was vanaf koringstrooi geïsoleer en gevolglik geanaliseer. Daar moes van die standaard prosedure vir die ekstraksie van lignienkoolhidraatkompleks afgewyk word ten einde beter lignienkoolhidraatkompleks opbrengs te lewer. Karakterisering van die lignienkoolhidraatkompleks na trifluoroasynsuurvertering was deur kapillêre elektroforese bepaal. Dit wou voorkom asof kapillêre elektroforese "n beter opsie vir die analise van die verteerde monster van lignienkoolhidraatkompleks is, vergeleke met hoëdruk vloeistof chromatografie. Daar was gevind dat die lignienkoolhidraatkompleks uit 68 % koolhidraat en 20 % lignien bestaan. Kapillêre elektroforese het die teenwoordigheid van die volgende suikers bevestig naamlik, mannose, xilose, arabinose, glukose, galaktose en ramnose. Mannose was die dominerende suiker in die lignienkoolhidraatkompleks wat 34 % van die totale koolhidraat opbrengs uitgemaak het. Ferulien- en p-kumaarsuur kon ook identifiseer word, maar die twee sure het minder as 1 % van die totale inhoud van die lignienkoolhidraatkompleks uitgemaak. Vanuit resultate bekom wil dit voorkom dat die lignienkoolhidraatkompleks "n heterogene molekuul is omdat die molekulêre gewig daarvan nie akkuraat bepaal kon word nie. 'n Geskatte molekulêre grootte van ongeveer 5.9 kDa was bepaal met verwysing na die hoogste piek wat in die chromatogram waargeneem was. Tien fungus kulture was in die studie gebruik om hul vermoë te toets om op die lignienkoolhidraatkompleks te groei, naamlik Aspergillus niger, Aureobasidium pullulans, Bjerkandera adusfa, Goriolus versicolor, Lenziius betuline. Phanerochaefe chrysosporium, Pycnoporus coccineus, Pycnoporus sanguineus 294, Pycnoporus sanguineus K5-2-3 en Trichoderma reesei. B. eauste het nie groei na 5 dae getoon nie, en dit wou voorkom asof A. niger, A. pul/ulans, G. versicolor, P. chrysosoporium en T. reesei die beste kon groei op die substraat na 5 dae. Die kultuurvloeistof van die vier kulture het getoon dat proteïene deur hierdie organisms uitgeskei was. Hierdie proteinbepaling was ook bevestig deur SOS-PAGE, wat bande tussen 8 kDa en 180 kDa gelewer het. Prominente bande tussen 26 kDa en 80 kDa kon waargeneem word in die kultuurvloeistof van A. niger, A. pul/ulans, en T. reesei, maar nie in die kultuurvloeistof van C. versicolor nie. Aktiwiteitstudies op die kultuur vloeistowwe het getoon dat daar aktiwiteit was vir die volgende ensieme, naamlik xilanase, xilosidase, arabinofuranosidase en feruliensuur esterase. Hierdie aktiwiteit het aansienlik verskil tussen die verskillende organismes. Die ensieme van die vier organismes was ook gebruik om hul vermoë te toets om xilaan en suikerriet af te breek. Daar was gevind dat xilaanafbraak verskillend was vir die organisms oor dieselfde tydperk. Xilotetraose, xilotriose, xilobiose en xilose was in verskillende konsentrasies gevind vir die verskillende organismes. Die elektroferogramme van die kultuurvloeistof op suikerriet van die verskillende organismes het getoon dat die substraat nie toeganklik vir die ensieme was nie. Die elektroferogramme van die kultuurvloeistof op lignienkoolhidraatkompleks van die verskillende organismes het dieselfde pieke getoon soos geïdentifiseer in die elektroferogramme van die trifluoroasynsuur vertering. Die elektroferogramme met die ensiem vertering het egter addisionele pieke getoon wat nie sigbaar op die elektroferogramme van die trifluoroasynsuur vertering was nie. Hierdie pieke het min of meer dieselfde tyd ge-elueer as die monosakkariede. Kapillêre elektroforese het ook getoon dat die ensiematiese afbraak van die lignienkoolhidraatkompleks gelei het tot algehele vrystelling van sommige suikers, wanneer die trifluoroasynsuur vertering as maatstaaf dien vir die hoeveelheid suikers teenwoordig in die lignienkoolhidraatkompleks. Hoëdruk vloeistof chromatografie het getoon dat geen sigbare afbraak na 72 h van inkubasie met die ensieme op die lignienkoolhidraatkompleks plaasgevind het nie aangesien die piek hoogtes konstant gebly het. Hierdie pieke kon egter nie geïdentifiseer word nie as gevolg van lae resolusie van die pieke asook standaarde wat nie beskikbaar was nie.

Densification of biomass is often necessary to combat the negative storage and handling characteristics of these low bulk density materials. A consistent, high-quality densified product is strongly desired, but not always delivered. Within the context of pelleting and briquetting, binding agents are commonly added to comminuted biomass feedstocks to improve the quality of the resulting pellets or briquettes. Many feedstocks naturally possess such binding agents; however, they may not be abundant enough or available in a form or state to significantly contribute to product binding. Also, process parameters (pressure and temperature) and material variables (particle size and moisture content) can be adjusted to improve the quality of the final densified product.<p>Densification of ground biomass materials is still not a science, as much work is still required to fully understand how the chemical composition and physical properties, along with the process variables, impact product quality. Generating densification and compression data, along with physical and mechanical properties of a variety of biomass materials will allow for a deeper understanding of the densification process. This in turn will result in the design of more efficient densification equipment, thus improving the feasibility of using biomass for chemical and energy production.<p>Experiments were carried out wherein process (pressure and temperature) and material (particle size and moisture content) variables were studied for their effect on the densification process (compression and relaxation characteristics) and the physical quality of the resulting products (pellets). Two feedstocks were selected for the investigation; namely, poplar wood and wheat straw, two prominent Canadian biomass resources. Steam explosion pretreatment was also investigated as a potential method of improving the densification characteristics and binding capacity of the two biomass feedstocks.<p> Compression/densification and relaxation testing was conducted in a closed-end cylindrical die at loads of 1000, 2000, 3000, and 4000 N (31.6, 63.2, 94.7, and 126.3 MPa) and die temperatures of 70 and 100°C. The raw poplar and wheat straw were first ground through a hammer mill fitted with 0.8 and 3.2 mm screens, while the particle size of the pretreated poplar and wheat straw was not adjusted. The four feedstocks (2 raw and 2 pretreated) were also conditioned to moisture contents of 9 and 15% wb prior to densification. <p> Previously developed empirical compression models fitted to the data elucidated that along with particle rearrangement and deformation, additional compression mechanisms were present during compression. Also, the compressibility and asymptotic modulus of the biomass grinds were increased by increasing the die temperature and decreasing product moisture content. While particle size did not have a significant effect on the compressibility, reducing it increased the resultant asymptotic modulus value. Steam explosion pretreatment served to decrease the compressibility and asymptotic modulus of the grinds.<p>In terms of physical quality of the resulting product, increasing the applied load naturally increased the initial density of the pellets (immediately after removal from the die). Increasing the die temperature served to increase the initial pellet density, decrease the dimensional (diametral and longitudinal) expansion (after 14 days), and increase the tensile strength of the pellets. Decreasing the raw feedstock particle size allowed for the increase in initial pellet density, decrease in diametral expansion (no effect on longitudinal expansion), and increase in tensile strength of the pellets. Decreasing the moisture content of the feedstocks allowed for higher initial pellet densities, but also an increased dimensional expansion. The pretreated feedstocks generally had higher initial pellet densities than the raw grinds. Also, the pretreated feedstocks shrank in diameter and length, and had higher tensile strengths than the raw feedstocks. The high performance of the pretreated poplar and wheat straw (as compared to their raw counterparts) was attributed to the disruption of the lignocellulosic structure, and removal/hydrolysis of hemicellulose, during the steam pretreatment process which was verified by chemical and Fourier transform infrared analysis. As a result, a higher relative amount of lignin was present. Also, the removal/hydrolysis of hemicellulose would indicate that this lignin was more readily available for binding, thus producing superior pellets.

Composites are combinations of mainly two different components: the matrix and the filler/reinforcement. In the thermoplastic composites industry, natural fibers from agricultural crops have been emerged as alternative fillers. Crops such as wheat straw are renewable and low cost materials that, combined with thermoplastics such as polypropylene, provide engineering products with unique characteristics. The objective of this study was to investigate the influence of processing conditions and composite formulation in the final properties of the composites. For these purposes wheat straw fibres and polypropylene (PP) were compounded in a batch mixer under a number of different thermal conditions and formulations. Fiber loading in the range from 0 to 60 wt-% was examined and the individual effects of two coupling agents (maleic anhydride modified polypropylene and maleic acid ethylene copolymer) and a lubricant were also studied. Particle size, morphology, thermal and mechanical properties and water uptake behaviour were inspected with appropriate techniques. Wheat straw particle size distribution was studied through image analysis; distribution curves for length and width of the particles were recorded in two stages of the project: previous and after compounding the natural material with polypropylene. Morphology of wheat straw particles and wheat straw-polypropylene composites were analyzed by scanning electron microscopy (SEM). Thermal properties including melting temperature and crystallization temperature of composites and pure resin were obtained from differential scanning calorimetry (DSC) performed on the samples; percentage of crystallinity was also calculated from the heat of fusion obtained from those tests. Mechanical properties, such as flexural modulus and flexural yield strength, were accessed in a miniature materials tester. Water absorption of selected composite samples was evaluated after immersion of the samples in a water bath. Water absorption curves were used to calculate the water diffusion coefficient (diffusivity) of the composites. Image analysis revealed the changes in the wheat straw structure due to shear forces during processing and improvement of adhesion between matrix and filler in compositions containing coupling agent. Small changes in the percentage of crystallinity of the thermoplastic phase were observed in all composites tested. Flexural tests revealed behaviour trends for the composites tested. Water uptake appeared to be a severe problem on natural fiber composites due to color fading, dimension instability and significant weight gains. Results from this work allowed the determination of some effects of processing temperature, fiber loading and use of additives on the final properties of wheat straw- polypropylene composites, thus making contributions to the scientific work that has been realized on natural fiber composites.

Composites are combinations of mainly two different components: the matrix and the filler/reinforcement. In the thermoplastic composites industry, natural fibers from agricultural crops have been emerged as alternative fillers. Crops such as wheat straw are renewable and low cost materials that, combined with thermoplastics such as polypropylene, provide engineering products with unique characteristics. The objective of this study was to investigate the influence of processing conditions and composite formulation in the final properties of the composites. For these purposes wheat straw fibres and polypropylene (PP) were compounded in a batch mixer under a number of different thermal conditions and formulations. Fiber loading in the range from 0 to 60 wt-% was examined and the individual effects of two coupling agents (maleic anhydride modified polypropylene and maleic acid ethylene copolymer) and a lubricant were also studied. Particle size, morphology, thermal and mechanical properties and water uptake behaviour were inspected with appropriate techniques. Wheat straw particle size distribution was studied through image analysis; distribution curves for length and width of the particles were recorded in two stages of the project: previous and after compounding the natural material with polypropylene. Morphology of wheat straw particles and wheat straw-polypropylene composites were analyzed by scanning electron microscopy (SEM). Thermal properties including melting temperature and crystallization temperature of composites and pure resin were obtained from differential scanning calorimetry (DSC) performed on the samples; percentage of crystallinity was also calculated from the heat of fusion obtained from those tests. Mechanical properties, such as flexural modulus and flexural yield strength, were accessed in a miniature materials tester. Water absorption of selected composite samples was evaluated after immersion of the samples in a water bath. Water absorption curves were used to calculate the water diffusion coefficient (diffusivity) of the composites. Image analysis revealed the changes in the wheat straw structure due to shear forces during processing and improvement of adhesion between matrix and filler in compositions containing coupling agent. Small changes in the percentage of crystallinity of the thermoplastic phase were observed in all composites tested. Flexural tests revealed behaviour trends for the composites tested. Water uptake appeared to be a severe problem on natural fiber composites due to color fading, dimension instability and significant weight gains. Results from this work allowed the determination of some effects of processing temperature, fiber loading and use of additives on the final properties of wheat straw- polypropylene composites, thus making contributions to the scientific work that has been realized on natural fiber composites.

Polypropylene continues to be the mainstream choice thermoplastic for automotive applications. In many applications PP is filled with mineral fillers for improvement of properties. Biobased natural fillers or fibres are attractive materials to reduce the weight because of the low specific gravity of the biobased materials compared to the mineral fillers. Our group has done extensive research on the development of wheat straw fiber in thermoplastics in the past years. It is very important to understand the behaviour of single fillers on composites before studying the effects of mixing fillers or fibers (hybridization). The objective of this study is to evaluate and compare systematically the effects of wheat straw and mineral fillers in the polypropylene matrix. The study includes two types of wheat straw (WS) categorized based on their size (fine WS and medium WS) and three different types of natural minerals (Talc, Mica and Wollastonite). Three types of polypropylene (PP), Homopolymer PP, High Impact Copolymer PP and Homopolymer-Copolymer Blend PP, were investigated as the matrix. This study also evaluates the effect of combining two fillers (WS and mineral filler) in the hybrid composite. The fillers were formulated in three different percentages (20, 30 and 40wt %) and compounded via extrusion. Samples for all formulations were prepared by injection molding. The mechanical properties (flexural modulus and strength, tensile modulus and strength, impact strength), water absorption and density were measured. The properties of hybrid composites were evaluated by varying the amounts of two fillers at 10wt%-20wt%, 15wt%-15wt% and 20wt%-10wt% each, keeping the overall filler content constant at 30wt%. The effect of type of filler, filler size and filler content were critical in this work. The results obtained from this study indicated that filler type and filler content greatly influenced the mechanical properties and water absorption characteristics of the composites. The flexural modulus increased with increasing filler content. It was interesting to observe that though the impact strength decreased with the addition of fillers, increasing the filler content from 20 to 40 wt% did not affect the property. With respect to all fillers, wollastonite improved the mechanical properties significantly. Increasing the amount of WS content reduced the composite’s resistance to water absorption. Among mineral fillers, mica showed significantly higher percentage gain in weight with water absorption. Combination of fillers at varying percentages did not have any synergy effect on the mechanical behaviour of the composite. The percentage increase in weight with water absorption was observed to be increasing with increasing WS content in hybrid composites, but significantly lower than pure WS composites. The morphological study on WS composites revealed improved interaction of filler with homopolymer and polypropylene blend.

The use of wheat straw and other agricultural by-product fibers in polymer composite materials offers many economical and environmental benefits. Wheat straw has been recently commercialized as new filler for polypropylene thermoplastic composites in automotive applications. However, to expand its application in the automotive industry and other sectors where highly-engineered materials are needed, a systematic database and reliable composite property models are needed. For this purpose, this research was systematically conducted. A product design approach is used in studying wheat straw polypropylene (WS-PP) composite. A set of thermoplastic composite specifications relevant to several automotive parts was used as a basis for the customer needs which give the direction to the entire product design of thermoplastic composites based on polypropylene and straw. Straw fibers were produced by grinding and sieving (without any other treatment). These fibers were used in the formulation of polypropylene thermoplastic composites to understand the variable that can contribute to minimize production cost, maximize product performance and maximize wheat straw utilization (fraction of renewable material). The variation in chemical composition due to plant variety (parts of the plant, location of harvesting and seasonality), the bonding incompatibility between hydrophobic polypropylene matrix and hydrophilic straw fiber, along with the heterogeneity of fiber size and shape, has made wheat straw polypropylene composite a complex system. This complexity causes the mechanistic approach of composite modeling in the well-established composite theory difficult to be applied, since modeling the contribution of natural fibers to the performance of thermoplastic composites is not as straightforward like in the case of homogenous glass fiber (with same shape, diameter and narrow length distribution). Alternatively, a statistical approach of modeling by using designed experiments was used in this research. The Mixture and Process-Mixture Experimental Design methodologies were applied to develop response surface models that can be used to correlate input properties and formulation of these thermoplastic composites to the final properties of the product. The models obtained can then be inverted to predict the required properties and formulations using fiber (straw), matrix (polypropylene), and additives (coupling agent) as the main components for a specified product performance. The prediction includes the fiber grading (size and aspect ratio) and classification in order to maximize fiber utilization for different needs of composite products. The experiments were designed based on the analysis of the existing data provided by previous research works of wheat straw polypropylene composite system in our laboratory and by experimental data generated during this research. The focus of the analysis was the determination of the factor(s), i.e., the independent variables of the experiments and their acceptable levels. The response variables being measured were chosen based on the required specifications of targeted products. A constrained three-component mixture design of experiment was conducted to develop models for flexural properties of WS-PP composite. The three independent mixture variables in this experiment were the weight proportions of: straw (as fiber), polypropylene (as matrix), and maleic anhydride polypropylene (as coupling agent). Statistical analysis results showed that the obtained models have met standard requirements of response surface models with good predictive capability. One of the important finding of this study was the formulation for optimum coupling agent proportion which gives the best flexural properties of composite. The effect of straw fiber size on composite properties was investigated by using fiber length and aspect ratio as parameters to describe fiber size, instead of the size of sieves used in fiber preparation. Two-stage separation method was applied in the straw fiber preparation process. In this method, width-based separation was followed by length-based separation to obtain fiber fractions with distinct fiber length and aspect ratio. Samples of thermoplastic composites for measurement of physical properties were produced from each fiber factions at two different levels of fiber loading. The samples were compounded by twin-screw extrusion and specimens were prepared by injection molding. The fibers were then extracted from the samples after injection molding (using solvent) and their sizes were measured to investigate the fiber size reduction during the compounding and molding process. A comprehensive analysis was then performed to study the responses of stiffness, impact resistance and specific properties of these composites by including initial fiber sizes, fiber chemical compositions (measured as cellulose, hemi-cellulose and lignin), fiber size reduction during compounding/molding process, and fiber loading as factors. One of the important contributions of this study is fiber grading in terms of their sizes and their respective contributions to the final composite product properties. Based on the previous results, a mixture design of experiment was performed on wheat straw – polypropylene / impact copolymer polypropylene (WS-PP/ICP) composite system. The objective of the experiment was to obtain response surface models that can be used to estimate some important properties required by a set of automotive product specifications. The optimum formulation of coupling agent obtained in the previous study was used to determine the fixed recipe of coupling agent; simplifying the composite system into a three-component mixture, i.e. straw (as fiber) and polypropylene (homopolymer and impact copolymer (polypropylene blend as matrix). Simulation of the models shows the superiority of using a blend of polypropylenes to balance the stiffness and impact strength of the composites and being able to reach three targeted product specifications. A case study was also performed to demonstrate that the models can be used to find optimum formulations to minimize material cost while meeting specifications of all targeted products. Finally, a framework for wheat straw polypropylene product design and development is presented in this thesis. The framework can be used for designing polypropylene-straw thermoplastic composites with various combinations of fiber - polymer matrix - additive systems with different product attributes and specifications suitable for several applications in the automotive industry.