Study On Composites From Rice (Wheat)Straw And Starch

Energy saving and environmental protection are the world’s urgent requirements. Development and utilization of agricultural waste and other renewable and degradable composites materials have become a research focus, which is environmental-friendly materials’ research direction for future development, and also an effective way of using crop waste and reducing environmental pollution. Straw and starch are both rich in resources, and their composites represented low density, light weight, low cost, renewable nature, and acceptable biodegradability, which other composites can not match. Research and applications of straw/starch composites have important significance for sustainable development, not only to ease the conflict of timber supply and demand, but also to prosper rural economy and increase farmers’ income.The main goal of this work was to use rice, wheat straw and corn starch, tapioca starch and potato starch as the main raw material to prepare environmentally sound, biodegradable composites using starch-based adhesives by a compression molding process. Effects of straw fiber pretreatments, straw fiber sizes, starch type, moisture resistance agent content, plasticizer content, and adhesive content on internal bond strength (IB), flexural strength (FS), modulus of elasticity (MOE), tensile strength (TS), modulus of rupture (MOR), elongation of fracture (EOF), impact strength (IS),2h thickness swelling (2hTS) and the moisture absorption rate (MA) of the obtained composites were investigated. Analysis and comparison of rice, wheat straw fiber morphology and properties were undertaken before and after pretreatments. The influence of temperature and shear rate variation on apparent viscosity of starch adhesive of different combinations had been also investigated. The main results obtained as follows:(1) Testing for tensile properties indicated that:pretreatments reduced the tensile strength of rice, wheat straw, and the biggest down-range appeared after NaOH treatment.(2) Infrared spectroscopy (IR) and scanning electron microscopy (SEM) suggested that:all pretreatments were efficient in partially changing RS surface properties, by removing wax-silicide layer on rice, wheat straw fibre surface in different levels, fully exposing the cell wall materials, as well as exposing functional groups of cellulose, hemicellulose, and lignin on the surface of straw, increasing the number of infrared absorption peak, enhancing the peak absorption, increasing water-holding capacity, improving the surface wettability. Among them, effects of NaOH treatment and hot-water treatment were more significant, while the oxalic acid treatment did not affect the basic organizational form of straw fibre significantly.(3) Moisture absorption of straw fiber showed that:moisture absorption rate increased with time increasing, and increased rapidly within6h. Moisture absorption rate of10-20mm straw section was smaller in the whole process, and their moisture equilibrium rates were0.9314%and1.071%. After crushing, rice, wheat straw fibers’ moisture absorption rate increased in different levels; after NaOH treatment, moisture absorption rate was largest, where2.243%and2.411%, respectively; hot-water treatment followed, where1.84%and1.714%, respectively; after oxalic acid treatment, where1.591%and1.514%, respectively; moisture absorption rate of untreated straw fiber was smaller, where1.585%and1.449%, respectively.(4) Wettability of straw fiber was also investigated and showed that:pretreatments of rice, wheat straw fiber lowered surface contact angle and enhanced wettability of distilled water on the straw surface. Hot-water treatment, NaOH treatment, oxalic acid treatment and untreated rice straw fibers’ contact angle at6min were71.5°,61.5°,88.5°and95.5°, and wheat straw fibers’were71.1°,56.5°,74.4°, and73.6°, respectively.(5) Differential scanning calorimetry (DSC) suggested that:pretreatments changed the glass transition temperature (Tg) of rice, wheat straw fiber, and decreased the contents of inorganic matter such as silicide.(6) Corn starch, tapioca starch and potato starch adhesives of starch to water mass ratio1/6,1/8and1/10were prepared. The influence of temperature and shear rate (decided by rotor speed) on apparent viscosity of starch adhesives was also investigated. Three starch adhesives showed a shear thinning phenomenon. Apparent viscosity decreased with starch/water mass ratio decreasing. The apparent viscosity of corn starch adhesive increased and then decreased with temperature increasing, and reached peak value at10℃. Apparent viscosity of cassava starch adhesive decreased with temperature increasing, changing significantly before15℃, and then easing. Apparent viscosity of potato starch adhesive decreased with temperature increasing. (7) The optimum molding process parameters of composites were pressure of4MPa, temperature of120℃, and pressure-holding time of30min, which were obtained by orthogonal test method L9(34) and flexural strength and flexural modulus as optimization parameters.(8) Based on the optimum molding process parameters, composites of different sizes straw fibers and different starch were prepared:(a), each property of composites changed differently with different dimensions, at0.2-0.45mm, IB was better; at0.45-0.9mm, FS, MOE were better; at0.9～30mm, TS, MOR, EOF and IS were better; at mixing straw fibres, the mechanical properties picked-up in different levels; at10-20mm section, only IS was better.2hTS and MA decreased and then increased with straw fiber size decreasing, at0.9～30mm,2hTS was bigger; at0.2～0.45mm,2hTS was smaller; composites from10～20mm section straw fibres dissolved in about0.5h.(b), IB, MOE, MOR and IS of corn starch-based composites were better, and EOF was smaller. FS and TS of cassava starch-based composites were better. Potato starch-based composites showed poor mechanical properties. The effect of starch types on2hTS and MA was slight.(9) FS, MOE, IS, TS, and MOR of composites from hot-water treatment straw fibers were higher, IB was lower; after NaOH treatment, IB was higher, FS was lower; after oxalic acid treatment, IB, FS, MOE, IS decreased in different levels. After NaOH treatment,2hTS was smaller. Pretreatments of straw fibers decreased moisture resistance properties of composites, and MA increased.(10) The introduction of moisture resistance agent reduced the mechanical properties in different levels, the maximum drops of IB, FS, MOE, TS, MOR, EOF, and IS were as follows:56.25%,6.68%,11.31%,40.17%,54.74%,20.4%and26.46%(rice straw fibers composites), and36.1%,56.2%,68.5%,56.5%,58.1%,20.3%and27.4%(wheat straw fibers composites).1%moisture resistance agent improved the performance of composites’ waterproof and moisture resistance.(11) The introduction of plasticizer reduced MOR and EOF slightly, but enhanced the other mechanical properties in different levels, while improved the performance of composites’ waterproof and moisture resistance.(12) The mechanical properties of rice straw fibers composites increased and then decreased with starch adhesive content increasing; IB, FS, MOE, TS, and IS reached peak value at10%adhesives content; IB and IS of wheat straw fibers composites reached peak value at9%adhesives content, while the other mechanical properties decreased with adhesives content increasing in different levels.2hTS reduced with starch adhesive content increasing slightly. The effect of adhesives content on MA was slight.(13) Microstructure of composites was also studied:all composites surfaces were smooth, but there are apertures, and gaps obviously in interface of matrix and reinforcement, which facilitated water or moisture to infiltrate inside of the composites. This also could explain why the moisture resistance was poor. The apertures and gaps become larger with the straw fibers size increasing. Otherwise, there were many cracks at<0.2mm.In summary, pretreatments of straw fibers changed surface properties or removed silicone-waxy layer partly, exposed the cell wall material, reduced cellulose, hemicellulose, and lignin content, increased water-holding capacity, improved surface wettability and moisture absorption, and reduced the surface contact angle, which provided a better combination interface of straw fiber and starch adhesives. Composites’ properties were decided by "competitive results" among the multiple factors. Under the same conditions, the apparent viscosity of different starch adhesives was quite different, but the relevant composites did not show similar performance as adhesives did. Straw fiber sizes, pretreatments and moisture resistance agents, plasticizers, adhesives content showed greater impact on performance of composites.