| Natural plant fiber reinforced biodegradable composites, which possess numerous advantages including environmental friendly, moisture resistance, noise insulation, shock absorption, and well mechanical strength, are attracting interest for a wide range of applications from special high-performance product in national defense and transportation through to commodity uses in civil engineering and packaging. However, there are several limitations like poor interfacial adhesion that constrain their utilization and service condition.In the present study, bamboo fiber (BF) reinforced polylactic acid (PLA) biodegradable composite was prepared. To improve interfacial adhesion, the BF was modified with 10wt% NaOH, and 1.5wt% MDI, respectively. Additionally, incorporated modification combining alkali pretreatment and MDI interfacial modifier (alkali/MDI) was applied. The effects of such treatments on the physical and mechanical properties for natural fiber and the obtained composite were investigated. The aging properties and biodegradation properties of resulting composite were studied. Based on the obtained results, the mechanisms involving interfacial modification and improvement, natural aging, and biodegradation of BF/PLA composite were revealed.Due to removal of pectin, extractives, and partial cellulose by alkali treatment, the crystallinity of bamboo fiber was increased, and porous structure displayed on bamboo fiber surface. Furthermore, the removal of fiber compositions increased available surface and enlarged micro-pores on fiber surface, and specific surface area increased by 45.9%. Chemical reaction between-N=C=O in MDIs and-OH on the fibers successfully grafted MDI modifiers onto cellulose molecules, resulting in a reduced polarity of bamboo fiber. As a consequence, the affinity between fibers and polymers which perform different polarity was significantly improved. The incorporation of alkali treatment and modifier has synergetic and integrated effects on the physical and chemical properties of bamboo fiber.The effect of interface modification on the interfacial adhesion, physical and mechanical strength, rheological and thermal properties, and water resistance of BF/PLA were characterized by using differential scanning calorimetry, thermal gravimetric analysis, capillary rheometer, scanning electron microscopy, and universal mechanical test. The results showed that, interfacial compatibility between BF and PLA was improved after such modifications, which resulted in an enhancement of tensile and impact strengths of obtained composite. Compared to untreated composite, the tensile strength of composite treated by alkali, MDI. and alkali/MDI were increased by 10.1%, 15.2%, and 36.9%, respectively. Correspondingly, the impact strengths were increased by 5.9%,14.1%, and 36.5%, respectively. The interfacial modification affected water absorption and equilibrium time of composite. Composite without any modifications exhibited largest water absorption and which reached equilibrium state in the shortest interval. Both composites treated by alkali and MDI had modest water absorption and equilibrium rate. However, composite treated by alkali/MDI absorbed least water but with lowest equilibrium rate. The viscous flow activation energy of BF/PLA composite was increased after interface modification. The highest and lowest activation energies were shown in composites treated by alkali and MDI. respectively. Because of the crosslinking between fibers and PLAs, the temperatures of pyrolysis, glass transition, and crystallization for composite after modifications were shifted to a high range. Owing to synergistic and integrated effects on physical and chemical modification, the incorporation treatment of alkali and MDI has preferable influence on improving comprehensive properties of resulting composite.The effects of fiber mass fraction (30%,40%,50%,60%) and MDI addition volume (0.5%,1.0%,1.5%,2.0%) on the performance of composite were studied, and composites were characterized by differential scanning calorimeter, thermal gravimetric analysis, capillary rheometer, gel permeation chromatography, scanning electron microscopy, and universal mechanical test. The results showed that, both tensile and impact strength increased along with the fiber mass increase, which decreased after reaching the peaks. When fiber mass fraction approached 50%, the tensile and impact strengths of composite obtained maximum values of 63.2 MPa, and 11.6 KJ/m2, respectively. The increase of fiber mass fraction caused a poor fiber distribution in polymer matrix and weakened interfacial adhesion, the tensile and impact strengths as a result were largely decreased. The MDI modifier was able to improve interfacial compatibility between fibers and polymers. The tensile strength of composite increased to 63.2 MPa when MDI was 1.5wt%, which is 52.7% higher compared to untreated composite behaving 41.4 MPa. Tensile strength of composite material was continually increased by increasing MDI addition, however, the increasing rate was significantly reduced, and gradually stabilized. The impact strength was increased by adding more MDI. After reaching the peak, it was significantly decreased. When the MDI addition is 1.5wt%, the impact strength reached maximum value of 11.6 KJ/m2, which is 70.6% higher compared to untreated composite that performs an impact strength of 6.8 KJ/m2. The increase of fiber mass fraction significantly increased 24h water absorption of composite. When fiber mass fraction is 30%, the 24h water absorption of the composite is only 1.2%. As a comparison, it was increased by 900% when fiber mass fraction increased to 60%. However, addition of MDI contributed to a decrease of water absorption for composites. The 24h water absorption of composites without MDI treatment was 10.2%, which was decreased to 5.1% after adding 2wt% MDI. The storage modulus of composites was increased by increasing fiber mass fraction. When the mass fraction is 50%, the storage modulus reached the maximum. The loss factor of composite was gradually reduced with the increase of fiber mass fraction. While, with the increase of MDI addition, the storage modulus of composite was gradually increased, and the loss factor peak was decreased. The apparent viscosity and viscous flow activation energies of composite were both increased by increasing fiber mass and MDI modifier. The thermal stability of composite was initially increased and then decreased with fiber mass fraction. While, composite containing 50%fiber mass fraction has the best thermal stability. MDI modification led to an increase of pyrolysis temperature, suggesting improved thermal stability of composite. With increase of fiber mass fraction, Tg and Tc temperatures of the composites were initially increased and then decreased, which reached highest when fiber fraction is 50%. Ta and Tc temperatures were increased along with increase of MDI addition.The natural aging, thermal aging, and their aging mechanisms of composite after incorporated modification was investigated by using differential scanning calorimetry, thermal gravimetric analysis, gel permeation chromatography, scanning electron microscopy and universal mechanical test. The results showed that, during thermal aging, the C=O groups in PLA molecular chain was hydrolyzed, and C-O groups was degraded, which resulted in decomposed PLA with lower degree of polymerization. Furthermore, the PLA was gradually transformed from crystalline state to amorphous, causing constant decrease of its mechanical strength. While, the pyrolysis of part of hemicellulose, oligomeric cellulose and lignin in bamboo fiber led to constant decrease of its mechanical strength. The tensile and impact strengths of composite decreased, causing by the degradation of fiber and polymer, and weakened interfacial adhesion between fiber and matrix. The tensile and impact strengths were reduced by 75% and 77.6% after thermal aging at 80 ℃ for 16 days, respectively. These indexes were reduced by 80.3% and 83.4% after thermal aging at 100 ℃ for 32 days, respectively. The processing temperature has significantly effect on aging performance of composite, in which a higher temperature led to a faster aging. In the practical manufacture, certain amount of anti-thermal aging agents should be added to improve the thermal aging properties composite. During natural aging, the synergistic effect among water, light, heat, and oxygen hydrolyzed PLA molecules, broke the molecular chain, decreased the molecular weight, transformed crystallinity from crystal region to amorphous, and led to a decreased mechanical strength. Under environmental condition, the interfacial adhesion between PLA and BF was declined by rainfall crushing of fibers, thermal expansion and cool shrinkage, and PLA decomposition. The declined interfacial adhesion resulted in reduced mechanical strength. The tensile and impact strengths of the composite were decreased by 69.6%, and 75.8% after 137 days natural aging, respectively. Several apparent cracks displayed on the surface of the composite after natural aging. Frankly speaking, the natural aging of BF/PLA composite was serious, and some special technical methods should be adopted in the further study, with intention to improve the outdoor performance.The natural degradation and its degradation mechanism of BF/PLA composite after incorporated modification were studied by soil burial test, and such properties were characterized by using differential scanning calorimetry, thermal gravimetric analysis, gel permeation chromatography, and scanning electron microscopy. The results showed that, when composite buried in the soil, the surface layer of polylactic acid was initially degraded, as a result, more water permeated into the inner layer of composite through micro-pores and interface crack between BF and PLA generated by the swelling and shrinkage of bamboo fiber. The polylactic acid was degraded stratified, and bamboo fiber was decomposed by decay fungi, resulting in significant mass loss in composite. The mass loss rate of composite reached 8.87% after 12-month degradation. In the natural degradation process, ester groups in polylactic acid molecule chain reacted with water, whcih declined molecular chain, broke order arrangement of PLA molecule chain, and decomposed crystal structure. Because of rapture of molecule chain, the average molecular weight of PLA was gradually decreased and the molecular weight distribution narrowed. The average molecular weight of the polylactic acid in composite was reduced by 25.9% after 12-month degradation. In the degradation process of composite, the PLA in inner layer was gradually degraded due to the destruction of interface between bamboo fibers and polylactic acid, and bamboo fiber was gradually decomposed by brown and white rot fungi, which resulted in reduced tensile and impact strengths. The impact and tensile strengths of composites were reduced by 44% and 43.8% after 12-month degradation, respectively. The composite became dark with rough surface, and partial nudity bamboo fibers were visible after 12-month degradation. The pure PLA buried in the soil was stratified degraded because of its dense structure, and its degradation rate is lower than that of composite. Therefore, its loss rates of mass, crystallinity, average molecular weight, and mechanical strength were smaller than composite. For instance, the mass loss rate of pure PLA was only 0.23%. The average weight of molecular was decreased by 15.3%. The impact and tensile strengths were decreased by 17.4%, and 17.2%, respectively. In a word, the natural degradation of BF/PLA composite in the soil is relatively slow, several special measures, such as targeted cultivate fungi and bacteria, or biological system should be taken into, to realize highly efficient degradation. |
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