Preparation Of Starch/ Maize Straw Fiber Composite Materials And Biomimetic Laminated Boards

With increasing uses of plastics, the environmental pollution from waste plastics has become a serious problem. Biodegradable materials have being developed for several decades aim at reducing pollution of plastic wastes. The plant fibers as natural polymers are abundant in source and have many special properties, such as high ratio of length to diameter, high specific strength, high specific surface area, low density and bio-degradation. Straw fiber is one of important plant fibers. Compared with other fiber reinforced composite materials, the straw fiber reinforced composite materials have unique advantages, such as lighter weight, lower price, good machinability and biodegradation.Corn stalk tegument separated from the corn pith was smashed to obtain the fibers. In order to investigate the bond properties between straw fiber and water-based adhesive, the contact angle between the outer surface or inner surface of straw bark and water was measured with a contact angle instrument. The result showed that the average contact angle of the outer surface was 92.2°, indicating that the outside surfaces of corn stalk is hydrophobic, and the average contact angle of the inner surface was 60.1°, indicating that the inner surfaces of corn stalk is wetting. The distribution of the fiber dimensions were measured and it was found that the fibers used for tests have 28.35%wt of 2mm～5mm in diameter, 9.29%wt of 1mm～2mm in diameter, 34.99%wt of 0.5mm～1mm in diameter and 27.36%wt of less than 0.5mm in diameter. The moisture content of the corn stalk fibers is 8.05%wt.The particle shapes of maize starch and yellow dextrin were analyzed with a microscopic image analysis system. The results showed that both the starch particles and the yellow dextrin particles display are round and polygon shape and there are concaves on the yellow dextrin particle surface. The particles of the maize starch adhesives are swelled when they are heated and, as a result, the crystalline structure of the starch particle of the starch-water suspension disappears. The rheological properties of three common maize starch adhesives prepared were examined with NXS-11A model rotational viscometer. It was found that the shear stress of the common maize starch adhesive is increased as the sheer rate is increased and the shear stress is increased with the increase of the ratio of starch with water under the identical sheer rate. The property data of the common maize starch adhesives were regressed by nonlinear regression method and their rheological models were established. The results showed that the common maize starch adhesives belong to a yield-pseudoplastic fluid.The cross-linking maize starch adhesives considering four main factors (water content, gelatinization temperature, NaOH as gelatinization agent and Na2B4O7·10H2O as cross-linking agent) with three levels were prepared based on an orthogonal test scheme L9(34) in order to increase the water resisting property and the bonding strength of the common maize starch adhesives. The bonding properties of the cross-linked maize starch adhesives were characterized using shearing strength under compression loading based on national standard of GB/T17517-1998. The experimental results were analyzed by range analysis method. The optimal levels of factors, the primary and secondary factors, the best combination of factors and their levels were obtained. The best combination of factors were tested and verified for the adhesive consisting of 10g starch, 125g water content, 80℃of gelatinization temperature, 0.10g of gelatinization agent and 0.03g of cross-linking agent. The compressive shearing strength was 1.425MPa for the optimal combination situation. The effects of all factors on compressive shearing strength were analyzed by variance analysis. The results showed that the water factor and the gelatinization temperature factor have obvious effect on the bonding properties of maize starch adhesives, while the gelatinization agent factor and the cross-linking agent factor have no obvious effects on the bonding properties of maize starch adhesives.The microstructure of the elytron materials of four species of insects, Protaetia orentalis, Copris ochus Motschulsky, Anoplophora chinensis and Cytister bengalensis Aube, were examined by scanning electronic microscopy (SEM). The results showed that the elytra are composed of laminated composite materials and the structures of their fibers or fiber bundles in have varied types, including T type structure, the branch fiber structure, the orthogonal laminated structure, the helicoidal structure, the hole structure and the reticular structure. The structural feature of the insect elytra material would provide some biomimetic models for the structural design of the biomimetic composite materials with light weight and high strength.Physical models of both branch fiber and the helicoidal fiber reinforced composites were established according to the microstructure analysis of the four species of insects'elytra. There are many types of fiber reinforced composite materials based on the different structures of biological composite materials, such as continuous fiber and the short fiber reinforced composites, unidirectional continuous fiber reinforced composites[0], two-directional continuous fiber reinforced composites [0/90], two-dimensional continuous fiber reinforced composites [0/90/0], two-directional continuous fiber reinforced composites [0/90/90/0], multidirectional continuous fiber reinforced composite [0/45/90/-45/0], multidirectional continuous fiber reinforced composite [0/45/90], unidirectional chopped fiber reinforced composites, flat random distribution short fiber reinforced composites and space random distribution short fiber reinforced composites.The composite materials, including the yellow dextrin/maize stalk fiber composite materials, the common maize starch/maize stalk fiber composite materials, the optimized maize stalk fiber composite materials, the cross-linked maize starch/maize stalk fiber composite materials, the yellow dextrin/maize stalk fiber biomimetic composite materials and the cross-linked maize starch/maize stalk fiber/continuous jute fiber biomimetic composite materials were prepared by press molding method. Their densities were determined. It was found from variance analysis that the effects of various factor on the density of maize stalk fiber composite materials was examined. The results showed that the pressure factor and the temperature factor during processing have evident effects on the density of the yellow dextrin/ maize stalk fiber composite materials, while the pressing time and the yellow dextrin content have no obvious effects on the density of the yellow dextrin/ maize stalk fiber composite materials. The density analysis of the cross-linked maize starch/maize stalk fiber composite materials showed that the NaOH has an evident effect on the density of the composite materials. The experimental results of the flexural properties of the yellow dextrin/maize stalk fiber composite materials and the cross-linked maize starch/maize stalk fiber composite materials were analyzed by range analysis method and variance analysis method, and the optimal level of factors, the primary and secondary factors, the best combination of factors and their levels were obtained. The best combination of factors were tested and verified. The maize stalk fiber biomimetic laminated composite materials were prepared learning from the structural models of the elytra material. The flexural strength and flexural elastic modulus of the biomimetic composites were examined. The results showed that the flexural strength of the single layer jute fiber reinforced maize stalk fiber composite materials and dual layer jute fiber reinforced maize stalk fiber composite materials are higher than the common maize stalk fiber composite materials and three groups of jute fiber hybrid reinforced straw fiber composite materials because of the biomimetic design.The hygroscopicity change regulations of the maize stalk fiber composite materials, including the yellow dextrin/ straw fiber composites, the common maize starch/maize stalk fiber composite materials, the cross-linked maize starch/maize stalk fiber composite materials, the optimized yellow dextrin/maize stalk fiber composite, the optimized cross-linked maize starch/maize stalk fiber composite, the yellow dextrin/maize stalk fiber biomimetic composites and the cross-linked maize starch/maize stalk fiber biomimetic composite materials, at different relative humidity conditions were revealed. The models of moisture absorption rate and their variation velocity were set up. The results show that the water-repellent property of yellow dextrin/maize stalk fiber composites can be improved with the increasing temperature during pressing. The water-repellent property of the cross-linked maize starch/maize stalk fiber composite materials is weak compared with other composite materials prepared by such resin adhesives as phenol formaldehyde resin and urea-formaldehyde resin. The maize stalk fiber composite materials possessing best water-repellent property at different relative humidity conditions were compared. The results show that the water-repellent property of the maize stalk fiber composite materials prepared with the starch adhesives by cross-linking treatment can be improved considerably.