English
Natural bamboo fiber is the most abundant natural polymer material in nature, with a total growth of billions of tons, far exceeding the total reserves of existing oil on Earth. In today's increasingly scarce natural resources, it is urgent to fully utilize natural bamboo fiber resources, leverage their unique functions and characteristics, and develop new application areas. The composite preparation of green composite materials using natural bamboo fibers and biodegradable plastics is one of the effective ways to develop and utilize this resource.
Natural bamboo fiber has many advantages such as high aspect ratio, high specific strength, large surface area, low density, low cost, renewability, and biodegradability, making it have good industrial prospects. The development and preparation of environmentally friendly and biodegradable green composite materials using natural bamboo fibers as reinforcement materials and biodegradable plastics as substrates has become a research hotspot in the new century.
However, the preparation process of natural bamboo fiber/biodegradable plastic green composite materials is relatively complex, and the properties of the composite materials are influenced by many factors such as raw materials, interface characteristics between natural bamboo fibers and biodegradable plastics, and molding processes. This article will analyze the factors that affect the performance of composite materials and elaborate on several key issues that should be noted in the research and preparation process of natural bamboo fiber/biodegradable plastic green composite materials.
1. Raw materials
1.1 Addition amount of natural bamboo fiber
At present, although research on the preparation of green composite materials from bamboo fibers and biodegradable plastics has been carried out, it is relatively less compared to the research on hemp fiber materials [3]. Regardless of which natural bamboo fiber is used to prepare green composite materials, the amount of bamboo fiber added is one of the main factors affecting the properties of the composite materials.
The influence of the amount of natural bamboo fiber added (fiber mass fraction) on the properties of composite materials is still controversial in the academic community. Some scholars believe that within a certain range, as the mass fraction of plant fibers increases, the mechanical properties of composite materials improve. Takagi et al. [4,5] conducted research on the molding process of bamboo fiber and biodegradable plastic green composite materials. Research has found that the tensile and bending strength of composite materials gradually increases with the increase of bamboo fiber mass fraction, and the material performance reaches its maximum value when the bamboo fiber mass fraction is 66.6%. But some scholars have different conclusions. In the bamboo fiber PPLA and bamboo fiber PPBS composite materials prepared by Lee et al. [6] using the melt kneading and hot pressing method, the tensile strength of the composite materials decreases with an increase in apparent warp content (from 10% to 50%).
The reasons for the above results are multifaceted. In addition to the performance of biodegradable plastics themselves, it is also related to the interface compatibility between natural bamboo fibers and biodegradable polymer plastics used, as well as the molding method of composite materials. Therefore, when conducting research on natural bamboo fiber/biodegradable plastic green composite materials, the selection of fiber mass fraction should consider the combined influence of multiple factors.
1.2 Properties of Biodegradable Plastics
Biodegradable plastics refer to plastics that are degraded in nature or under specific conditions such as composting conditions, anaerobic digestion conditions, or aqueous culture media by microorganisms such as red bacteria, molds, and seaweed, and ultimately completely degraded into carbon dioxide or mineralized inorganic salts with methane, water, and their contained elements, as well as new biomass. Currently, successful research and commercial production mainly include polylactic acid (PLA), polybutylene succinate (PBS), polyhydroxybutyrate (PHB), polycaprolactam (PCL), etc. Most of these plastics have excellent plasticity and are easy to process and shape, but there are also some shortcomings. For example PLA has a high brittleness, which can have a certain impact on the processing performance and toughness of composite materials. It is best to add a certain amount of plasticizer for modification when using it. Hu Xiaofang et al. [8] studied the modification of PLA with triacetin as a plasticizer and found that its toughness was enhanced without changing the crystal structure of PLA. In addition, the high production cost of these biodegradable plastics also restricts their application in green composite materials. Developing low-cost biodegradable plastics that can balance their processability, degradability, and mechanical properties is an urgent task.
2. Improvement of Interface Properties between Natural Bamboo Fiber and Biodegradable Plastic
Natural bamboo fiber is a natural polymer material composed of cellulose, hemicellulose, lignin, and various extracts. It is an uneven anisotropic material with complex interfacial properties. Its main components, such as cellulose, hemicellulose, and lignin, contain a large number of polar hydroxyl and phenolic hydroxyl functional groups, resulting in strong chemical polarity and poor interfacial compatibility between natural bamboo fiber and biodegradable plastic substrates. At the microscopic level, it presents a non-uniform system with a very clear interface between the two phases and poor adhesion. This makes it difficult for stress to be effectively transmitted at the interface, resulting in a significant decrease in the impact strength and tensile strength of the composite material, thereby affecting its overall performance [10.11]. Therefore, in the process of preparing green composite materials, the major problem that needs to be solved is how to achieve good compatibility between the hydrophilic polar natural bamboo fiber surface and the hydrophobic non-polar biodegradable plastic interface, so as to achieve intermolecular fusion between the surface layer of bamboo fiber and the surface layer of biodegradable plastic, and composite these two materials with different properties together to produce new materials with better performance than the original single material. Improving the compatibility between natural bamboo fibers and biodegradable plastics can draw on the improvement of the interface between natural bamboo fibers and ordinary biodegradable plastics. At present, the most commonly used methods are alkali treatment of bamboo fiber, coupling agent modification of plant fiber, and plastic modification.
2.1 Treatment of bamboo fiber with alkaline solution
The principle of alkali treatment of bamboo fibers is similar to that of other natural plant fibers, mainly manifested in two aspects: utilizing the different stability of alkali in various components of plant fibers to dissolve some impurities such as pectin, lignin, and hemicellulose in plant fibers, making the fiber surface rough and enhancing the mechanical adhesion between fibers and plastic interfaces. On the other hand, the plant fibers are refined through the action of alkaline solution, resulting in a decrease in fiber diameter, an increase in aspect ratio, and an increase in effective contact surface with the plastic matrix. Wang Junbo et al. [10] prepared ramie fiber-reinforced resin composite materials and found that alkali treatment can dissolve and expand the fiber pores. It is beneficial to fill in other materials to make the fibers into composite fibers, while also improving the tensile strength, tensile modulus, and toughness of the fiber body. The interface between fibers and resin was improved after alkali treatment, enhancing the mechanical properties of the composite material. Cao Yong et al. [12] prepared degradable composite materials using sugarcane bagasse fibers treated with different concentrations of alkaline solution as reinforcement materials, and studied the effect of alkaline treatment on the composite materials. Research has found that different concentrations of alkaline solution have a significant impact on the mechanical properties of fiber composite materials, with 1% being the most suitable concentration for alkaline treatment. After treatment with 1% alkaline solution, the tensile strength, bending strength, and impact strength of the fiber composite material increased by an average of about 13% compared to before treatment. The scanning electron microscope photos clearly show that there is a large amount of hemicellulose on the fiber surface before alkali treatment. After alkali treatment, due to the removal of hemicellulose, the fiber surface becomes clean and the unevenness is obvious, and the fiber gap becomes larger. This is similar to the effect of increasing the number of fibers, which also improves the bonding performance between the fibers and the substrate. At the same time, in order to further quantitatively analyze the effect of alkali treatment on fibers, he also conducted tensile performance tests on sugarcane bagasse fibers before and after alkali treatment. The test results showed that alkali treatment refined the fiber decomposition, resulting in an increase in fiber tensile strength and aspect ratio, thereby correspondingly improving the mechanical properties of materials prepared from alkali treated fibers.
2.2 Coupling agent modification treatment
Coupling agents are generally compounds with polar groups on one end and non-polar groups on the other end. The polar end is partially compatible with plant fibers, while the non-polar end is partially compatible with plastics, thus serving as a bridge between the two phases and connecting them together. The coupling agent method is to pre treat bamboo fibers with coupling agents before compounding with plastic, in order to improve the compatibility between bamboo fibers and plastic. The coupling agents currently used mainly include maleic acid series and aliphatic series. The dosage of coupling agent has a significant impact on the mechanical properties of composite materials. Lee et al. [13] evaluated the improvement effect of PLAPBS bamboo fiber bio composite material using lysine based isocyanate (LDI) as a coupling agent. The study found that after adding LDI The tensile properties, water resistance, and interfacial bonding properties of PLA/BF and PBS/BF composite materials have been improved.
2.3 Modification of plastics
Modifying biodegradable plastics as green composite materials is another important way to improve the interfacial properties between natural bamboo fibers and bioplastics. The main principle is to modify biodegradable plastics by grafting polar or reactive branches onto the polymer chains of plastics to improve the interfacial compatibility between bamboo fibers and biodegradable plastics. This modification includes the improvement of plastic properties and surface properties. From the perspective of improving the interfacial strength between natural bamboo fibers and biodegradable plastics, the improvement of plastic surface properties should be the main consideration, which is similar to the preparation of other plant fiber-reinforced biodegradable plastic composite materials. Plackett [14,15] modified polylactic acid with maleic anhydride esterification as a substrate for preparing wood fiber reinforced composite materials. The research results showed that maleic anhydride esterification modified polylactic acid had a positive effect on improving the interface of dry composite materials.
Selection of 3 molding processes
Similar to the preparation of natural bamboo fiber and ordinary polymer composite materials. The main molding processes used in the preparation of natural bamboo fiber/biodegradable plastic green composite materials include extrusion molding, injection molding, and lamination molding. Extrusion molding has a short processing cycle, high efficiency, and simple molding process, making it more widely used in industrial production compared to other processing methods. Extrusion molding requires granulation treatment of raw materials before molding processing, that is, the mixture of bamboo fiber and biodegradable plastic is processed into granules through a granulation device. The purpose is to melt and mix plant plastic mixture, pre plasticize bamboo plastic, eliminate volatiles, so that it can be fully plasticized during extrusion molding. The control of temperature, pressure, and screw speed is crucial in the extrusion molding process. Plant fibers are a type of rigid material that, when added to a plastic matrix, increases the viscosity of the mixed system. With high viscosity, bamboo fibers tend to agglomerate in the melt. The increase in temperature is beneficial for the flow of the molten system, but excessive temperature can cause degradation and coking of plant fibers, resulting in a decrease in the mechanical properties of the product and a darker appearance color. Generally, bamboo fibers begin to degrade and char above 200 degrees Celsius, so the set temperature is generally higher than the melting temperature of the plastic matrix and lower than 200 degrees Celsius. In addition, when the molten system of bamboo fiber and plastic reaches the mouth mold, a certain pressure must be ensured. Without sufficient extrusion pressure, it will cause strength defects in the product and is not conducive to the shaping of the product during extrusion [16,17]. The advantages of injection molding are fast production speed, high efficiency, easy implementation of automated production, high product size stability, and the ability to form complex shaped products. However, the proportion of bamboo fiber used as a reinforcing material during injection molding should not be too large, as the more fibers there are, the poorer the flowability of the mixed system, and the injection molding raw materials require high flowability. In addition, due to the lack of exhaust devices in injection molding, sufficient drying of raw materials is crucial [18,19].
Immerse bamboo fibers in a solution of biodegradable plastic, coat the surface of the bamboo fibers with a layer of plastic, and then pre press them into a plant fiber film or sheet. The laminating process is a molding process in which pre pressed bamboo fiber films or sheets are stacked to form a layered structure, which is then fed into a press and cured under heating and pressure to form composite material products. The laminated molding process is suitable for the production of composite materials with high bamboo fiber content, and its technical feature is that the pressure direction is perpendicular to the direction of the product surface. The laminating process mainly produces composite material sheets of various specifications and uses. It has a high degree of mechanization and automation, and the mechanical properties of the product are better than other molding methods, but the equipment investment is large at once [20-23].
The molding process of natural bamboo fiber/biodegradable plastic green composite materials should be selected according to the specific research content of the researchers. Overall, the following issues should be noted: (1) The molding of natural bamboo fiber/biodegradable plastic is more difficult than that of natural bamboo fiber/ordinary plastic composite materials. Because some biodegradable plastics are prone to hydrolysis reactions or may experience a decrease in molecular weight during extrusion and injection molding processes at temperatures as high as 200 degrees Celsius, which can lower the performance of composite materials. (2) According to available literature, composite materials produced by the lamination process have higher mechanical properties [25]. (3) Extrusion molding and injection molding processes are suitable for bamboo fibers to appear in powder form, and the dimensional mass fraction cannot be too high, because these two molding methods require the material mixing system to have a certain viscosity and flowability [18-21].
4 Conclusion
Natural bamboo fiber/biodegradable plastic composite material is a new type of environmentally friendly biomass composite material. Its application and development in China are mainly in the experimental research stage, and further research is needed to truly expand its large-scale development and application. Its research involves many disciplines such as material interface science, rheology, materials science, and material mechanics. It requires the comprehensive use of various modern analytical methods to systematically study the composite laws between bamboo fibers and biodegradable plastics, such as interfacial chemical bonding, interfacial tissue morphology, interfacial acid-base reactions, surface wetting, and other phenomena. At present, although there are still various problems in the research of natural bamboo fiber biodegradable plastic composite materials, with the reduction of available petroleum resources and the increasing awareness of environmental protection, people will pay more attention and importance to the development and application of this material, and its preparation theory and technology will become increasingly perfect and mature.
