Disposal of plastic wastes is a serious environmental problem that we face today. Mass production and increased use of plastics in wide applications in our daily life1,2 have resulted in environmental impact. Consequently, these issues lead to the growing threat of global warming resulting from carbon dioxide emission due to burning of non-biodegradable conventional polymers such as polyethylene, polypropylene, and polyvinylchloride3. Biodegradable polymers are being developed to be used as an alternative for non-biodegradable polymer materials in a variety of applications4. The best option for managing non-biodegradable plastic waste is to replace the use of uneconomical non-biodegradable materials for recycling or reuse with biodegradable polymers as they are environmentally friendly5. Because of the environmental pollution resulting from the use of non-biodegradable materials, studies and developments have increased about biodegradable materials6.
Biodegradable polymers are materials that can work for a limited time before degrading into readily discarded products through a regulated procedure7. They might be made from a variety of wastes or/and bioresources, such as wastes of food, animal, agro-waste as well as other sources such as starch, and cellulose. Bioplastics made from renewable resources are often less expensive than those made from microbial resources prompting producers to concentrate on making bioplastics from renewable resources8. The use of biodegradable polymers has environmental benefits such as regeneration of raw materials, biodegradation and reduction of carbon dioxide emissions that are led to global warming9. Microorganisms such as bacteria and fungus may consume biodegradable polymers and convert them to H2O, CO2, and methane. The biodegradation process depends on the material’s composition10. The polymer morphology, polymer structure, chemical and radiation treatments, and polymer molecular weight are all parameters that influence the biodegradation process11. Biodegradable polymers are also called biopolymers12. There are two reasons to use the polymers from renewable resources; (i) environmental concerns in terms of increasing plastic waste and global warming as a result of releasing of carbon dioxide when burning waste, and (ii) petroleum resources are limited and ended11.
The biodegradable plastics industry is highly promising. However, they need to be developed in tandem thorough examination of end-of-life processes of treatment and a worldwide integration with organic management of waste as selective biowaste collection expands. Biodegradable plastics have the benefit of being able to be handled biologically at the end of their lives (composting or anaerobic digestion). Composting of biodegradable plastics is thoroughly documented and recognized on a global scale. Product’s biodegradability can be conducted under home or industrial composting conditions. In the absence of oxygen, anaerobic digestion transforms organic waste into biogas (a mixture of CO2 and CH4), which may then be additionally valorized through heat and power generation13. Developing alternative and targeted waste management solutions are also a major challenge to encourage the integration of biopolymers into the circular economy14. Biodegradable polymers may be categorized in step with their origin11. To manage non-degradable waste and reduce its accumulation in the environment, biodegradable materials have been replaced nonbiodegradable materials in several uses such as plastic bags and packaging. In the field of human health care, biodegradable polymers are also used in various vital applications like soft tissue engineering, gene therapy and soft tissue engineering15.
The required properties and performance can be provided at a reasonable cost by mixing the matrix of polymers with natural fibers to form natural fiber-reinforced composites (NFRCs). Natural fibers can provide many environmental and economic advantages including being renewable and accessible in large quantities and reduce the cost of raw materials. They can be recycled, biodegradable, achieve a large stiffness to weight ratio and high thermal insulation. Natural fibers are as inexpensive as synthetic fibers such as carbon and glass fibers. Producing 1 tonne of continuous filament glass fiber products from raw materials for factory export emits between 1.4 and 2.2 tonnes CO2-eq, with an average of 1.8 tonnes CO2-eq. Compared to natural fibers which emit between 0.3 and 0.7 tonnes of CO2-eq per tonne of natural fiber16. Natural fibers may be categorized in step with their origin into plant fibers such as flax and cotton. Animal fibers can be extracted from wool and hair. Mineral fibers consist of silicates such as asbestos, inosilicates, and clay17,18. Although mineral fibers are obtained from nature, they are non-renewable materials17. Because of the hydrophobic polymers character and hydrophilic character of the natural fibers, the compatibility between them is very low and a major challenge19. Therefore, many techniques are applied to treat and modify the natural fiber surfaces to decrease absorption of moisture and enhance adhesion to the matrix of polymers. These treatments can be chemical, physical, or biological treatments. There are many factors that affect the changes in the chemical composition, properties, and structural aspects of fibers such as the origin of growth, the age of the fiber plant, climatic conditions, and different extraction patterns17. For clarification, some properties and chemical composition of several fibers will be mentioned in this research.
In this article some properties of biodegradable polymers, natural fibers, and biocomposites were defined. These properties include mechanical properties, thermal properties, visco-elastic behavior, acoustic properties, surface morphology and chemical composition of materials. The thermal properties of materials were examined by using thermal gravimetric analysis. The visco-elastic behavior was reported using dynamic mechanical analysis. The chemical composition of the composite and the functional groups which are responsible for the reaction within natural fibers and provide information about covalent bonding were identified by using Fourier-transform infrared spectroscopy.
Synthetic polymers have become necessary for several applications in daily life. They are non-biodegradable materials, so they accumulate in the environment causing serious problems. Biodegradable polymers are now gaining attention as an alternative to conventional polymers. Biodegradable polymers are materials that can be degraded naturally by the action of microorganisms producing eco-friendly and useful materials such as CO2 and CH4. Blending two or more biopolymers and mixing with natural fibers surface modification are used to improve the mechanical and physical properties of biodegradable polymers. Biopolymers can be used in a wide range of applications such as plastic bags, parts for automotive and medical applications. Biodegradable materials have the advantage of being able to be composted with organic waste or utilized to generate biomass at the end of usage of these materials. Also, biodegradable materials can be recycled to produce useful monomers and oligomers by microorganisms and use to produce the origin products. Biopolymer failures have been explained according to forensic engineering to try to avoid or reduce these failures during the manufacture and use.
The environment suffers from serious problems from the increasing difficulties of disposing of plastic waste that resist microbial degradation23. Therefore, the researchers tried to produce biodegradable, non-polluting, and environmentally friendly materials24. In recent times, the natural and synthetic origin of biodegradable polymers was produced with good compatibility and biodegradability11. As the biodegradable polymers receive great attention because they degrade into non-toxic and environmentally friendly materials24. Mechanical strength, thermal and electrical properties of common biodegradable polymers and their composites are shown in Fig. 2a–c. Polyglycolide (polyglycolic acid, PGA) has high tensile strength (70–117 MPa). Thermoplastic starch showed low tensile strength (16–22 MPa).
Polylactide (PLA)—glass fiber composites have high electrical resistivity (2.5 × –4.9 × ) µohm.cm as shown in Fig. 2b. Melting temperature of biodegradable polymers is variant. Polyglycolide has high melting temperature (220–231 °C) and high Young’s modulus (6.1–7.2 GPa) as shown in Fig. 2c.
Classification of biodegradable polymers
Biodegradable polymers may be categorized based on their origin and synthesis method, their chemical composition, economic importance, processing method and applications. They are categorized in this study based on their origin24. Biodegradable polymers are classified into two groups based on their origin as indicated in Fig. 3. Natural biopolymers and synthetic biopolymers are made from natural resources and oil, respectively11,18. Natural biopolymers are derived from renewable or biological sources such as animal, plant, marine, and microbial sources, while synthetic biodegradable polymers are manufactured chemically25.
Natural biodegradable polymers
All organisms’ growth cycles result in the formation of biopolymers in nature. Polymerization reactions with a chain of enzyme-catalyzed growth from active monomers generated within cells through complicated metabolic processes are included in its production23. They are naturally biodegradable and have good biocompatibility26.
Biopolymers directly extracted from biomass
Agricultural waste is a major source for the production of bioplastics, plasticizers and antioxidant additives. The main source of polysaccharides is plant-based agricultural waste where biopolymers such as cellulose, starch, and pectin are produced27,28. The use of agro-waste as a feedstock for biodegradable polymer synthesis can reduce both the cost of producing biodegradable polymers and the waste treatment cost. Biopolymers are produced through several methods, namely microbial methods, blending of biopolymers, and chemical methods27. Biopolymers produced from agricultural plant waste have biodegradability, bio-functionality, biostability, and biocompatibility. They have wide range of chemical and mechanical properties that may be employed in several applications such as food packaging, biomedical applications, skincare, electrical electronics, vehicles and wastewater treatment28.
Polysaccharides
Polysaccharides, proteins, and lipids are found in numerous applications in biodegradable products. Potatoes, corn, and rice are basic sources of starch production, where the chemical composition and granules size of starch varies according to the source of production11. As shown in Fig. 4a, starch is a combination of linear (amylose) and branching (amylopectin) poly-(1,4) -α-glucose29. The ratio of amylose to amylopectin has a substantial impact on the physicochemical characteristics of starch30. Starch has poor mechanical properties, low impact resistance, water sensitivity, and brittleness properties. The properties can be improved by reinforcing the starch matrix with fibers or modifying the starch chemically or physically with other biodegradable polymers to enhance its properties24,31. Starch is not completely soluble in water. It is partially soluble in water at room temperature, depending on the proportions of amylose and amylopectin32,33,34. There are many advantages to starch biopolymers, such as high biodegradation, renewability, and good oxygen barrier properties that make them suitable in some commercial applications such as packaging applications, bags, cosmetics, adhesives, medical applications, and agricultural mulch films3,30,35.One of the most common biopolymers is cellulose29. It contains reactive OH groups in the backbone31. Cellulose is a polysaccharide with a molecular structure that is similar to starch. However, d-glucose units are attached to β-glycosidic bonds in cellulose. In starch, d-glucose units are linked to -α-glycosidic bonds. As illustrated in Fig. 4b, cellulose consists of polymer chains made up of unbranched β (1 → 4) connected D-glucopyranosyl units29. Cellulose is the primary component of lignocellulosic plant cell walls. Hemicellulose, lignin, and other carbohydrate polymers are components of a gel matrix embedded with cellulose which exists in the lignocellulosic materials11. Cellulose and cellulose derivatives are ecologically friendly materials that are widely utilized due to their ability to decay, compatibility with other materials, and regenerate. Because of cellulose’s hydrophilic nature, moisture absorption causes the material’s mechanical characteristics to deteriorate. As a result, cellulose derivatives are created through chemical modification, in which bonds are formed between the reagents and the OH groups in the cellulose improving solubility and mechanical characteristics by reducing cellulose’s hydrophilicity31. Cellulose and its derivatives are used in different applications such as fiber industries, wood products, textile, and pharmaceutical industries3.
Chitin is a polysaccharide can be produced by fungal fermentation processes or found in marine crustaceans such as shrimps, insects, the shells of crabs and lobsters. Chitosan is a deacylated chitin derivative that makes up the exoskeleton of arthropods11. Chitosan is a derivative of glucan that is produced by repeated monomers of chitin. Biodegradability, high strength, and nontoxicity are among the chemical and physical properties of chitosan and chitin36. They have several applications in the field of medical and pharmaceutical sciences, tissue engineering applications, absorption of heavy metal ions, membrane barriers, the cosmetic industry and food packaging material3,36.
Polypeptides (Proteins)
Proteins are natural polymers produced from various vegetables and animals. Collagen is a protein found in both vertebrate and invertebrate connective tissue. It accounts for around half of all human protein. Collagen has a hydrophilic nature as a result of the enormous substance of essential, acidic, and hydroxylated amino corrosive buildups that lipophilic deposits11. Collagen properties are exceptionally compelling in many fields, for example, leather chemistry, surgery, pharmaceutical industries like capsule production, etc.3,37.
Biopolymers derived directly from naturally occurring or genetically engineered organisms
Microorganisms are a source for research on bioplastic materials and biopolymers that use agricultural wastes as a medium for growth. Example of microbiological compounding plastic is polyhydroxyalkanoate (PHA), which is produced from various groups of bacteria and cheap renewable resources. It is completely aerobic decomposed by microorganisms30. polyhydroxyalkanoates are produced from a biodegradable aliphatic polyester family. They are formed in nature and produced directly by bacterial metabolism38,39. They are genuinely biodegradable and profoundly biocompatible thermoplastic materials and can be developed from a variety of renewable resources. Polyhydroxyalkanoate is the potential alternative to the non-degradable polyethylene and polypropylene. Poly(3-hydroxybutyrate) (PHB) is the most representative member of this family11,38,39. It is a natural polymer that formed by various chains of bacteria. It is produced from low-cost, renewable feedstock and without causing much impact on the environment. It is degraded in anaerobic and aerobic conditions and it doesn’t produce any poisonous materials from its degradation22. Copolymer of hydroxybutyrate (PHB) and hydroxyvalerate (HV) is known as poly(3-hydroxybutyrate)- hydroxyvalerate (PHBV). It is a highly crystalline polymer. By increasing the HV unit content, the melting point, the glass transition temperature, the crystallinity and the tensile strength decrease but the impact strength increases. The copolymer PHBV is less brittle than PHB. The degradation rate of PHBV is higher than PHB40,41.
Synthetic biodegradable polymers
Synthetic biodegradable polymers are produced by conventional polymerization procedures such as aliphatic polyesters, polylactide, aliphatic copolymer42. Due to the appropriate degradation time and their production in industrial scale, the degradable polyesters are considered as the most potential materials utilized comparing to conventional plastics. They are produced in several forms such as polylactic acid, polycaprolactone and polybutylene succinate. They have low ecological contamination43. More than 90% of the biopolymers are polyester as they contain easily hydrolyzable bonds of esters. Synthetic biodegradable polymers may be categorized into bio-based polymers like PLA and oil-based monomers such as PCL12,42.
Aliphatic polyester synthesized from bio-derived monomers (water-based monomers)
The most significant biodegradable polymer is polylactic acid (PLA)3. It is well-known for being biodegradable and biocompatible polymer29. It is synthesized from renewable materials such as potato starch, wheat, rice bran corn and biomass22,31. Polylactic acid is linear aliphatic polyester thermoplastic polymers22. Polycondensation and ring-opening polymerization of lactic acid monomers are two processes that may be used to synthesize PLA3. PLA’s high molecular weight is achieved by polymerization of ring-opening. The final properties of the polymer can be controlled using this method44. Lactic acid monomer exists in three diastereoisomeric structures which are D-lactide, L-lactide, and meso-lactide (DL-lactide)11. Mechanical, thermal and biodegradable properties of PLA dependen on the selection of stereoisomers distribution within the polymer chain. PLA is used in many applications such as interference screws, sutures, dental, ligating clips, bone pins and rods3.
Polyglycolic acid (PGA) is a linear aliphatic polyester that is synthesized by polymerization of ring-opening of cyclic glycolide monomers12. It is a thermoplastic and biodegradable polymer. The degradation process of this polymer takes place in two stages. In the first one, water is dispersed in the amorphous regions of the polymer matrix where the ester bonds are separated. In the second stage, crystalline segment of the polymer gets vulnerable to hydrolytic attack11. This polymer is used in several applications such as tissue engineering, medical devices, and drug delivery3. Glycolic acid monomers (GA) are copolymerized with L-lactide and LD-lactide (LA) generating poly(lactide-co-glycolide) (PLGA). The rate of degradation decreases when the L/G proportion increased45. Mechanical and chemical properties are controlled by adjusting the ratio of monomers in the combined polymerization of the PLA and PGA without affecting the compatibility and biodegradability. The degradation of the PLGA takes place by hydrolysis of the ester bond and to form LA and GA which are eco-friendly materials31.
Aliphatic polyester synthesized from synthetic monomers (petroleum-based monomers)
Polycaprolactone (PCL) is a biodegradable synthetic linear aliphatic polyester. It can be synthesized by polymerization of ring-opening of caprolactone monomers within the sight of metal alkoxides catalysts. Aluminum isopropoxide and tin octoate are common catalysts used in polymerization process22,46. Polycaprolactone is dissolved in numerous solvents such as methylene chloride46. The melting point of PCL is between 58-60 °C. It is degraded by the activity of anaerobic and aerobic microorganisms. The degradation rate of PCL depends on the level of crystallinity and its molecular weight22. This polymer is used in medical fields such as tissue engineering, long-term drug and vaccine delivery vehicles3. The biodegradability of PCL can be improved by copolymerization with aliphatic polyesters. PLA and PGA copolymer has lower crystallinity and melting point compared with the homopolymer22,45,46.
Polybutylene succinate (PBS) can be prepared by polycondensation of 1,4 butanediol with aliphatic dicarboxylic acid succinic acid47. It is a thermoplastic aliphatic polyester that has a melting point between 90–120 °C11,47. It is decomposed by a hydrolysis mechanism22. Because PBS is crystalline materials, it degrades slowly11. It is normally blended with different materials such as adipate copolymer which acquiring polybutylene succinate adipate (PBSA)22. This copolymer has a higher degradation rate than PBS11.
Biodegradable polymer blends and composites
The main characteristics of biodegradable polymers are their primarily hydrophilic nature, high rate of decomposition, and potentially undesirable mechanical qualities. These properties could be improved by mixing natural and synthetic polymers11,48. Blending polymers is a category of substances where two polymers at least are mixed collectively to produce a new material with various physical properties. The purpose of mixing two or more polymers is to develop a blend that combines each polymer’s preferred properties49. The preparation of biodegradable polymer blends is normally associated with the blending of a thermoplastic resin with biodegradable one. These types of blends are expected to be more biodegradable than conventional plastics50. After the degradation process of the biodegradable materials, the residuals components are more ecologically friendly and do not cause environmental pollution3.
Natural fibers are perfect reinforcing materials for polymer composites (thermoplastics, thermosets, and elastomers). Natural fiber-reinforced polymer composites are gaining popularity due to their excellent mechanical properties and considerable manufacturing benefits, as well as the fact that they provide a solution to environmental contamination. Composites based on natural fibers have better impacts in the industry due to these fibers have high specific properties and low density. They don’t have health hazards because of they are non-toxic51. Natural fibers are renewable materials, relatively high in tensile strength, low-cost, and light52. Natural fibers have become a substitute for non-renewable and expensive synthetic fibers (glass, carbon and kevlar fibers)53 in different applications because of environmental aspects and their properties as shown in Fig. 554. Natural fibers contain some desirable properties, including high specific strength and modulus, flexibility during treatment, and excellent corrosion resistance53. However they show some limitations such as high anisotropy, moisture absorption, limited compatibility with conventional resins, and inferior homogeneity when compared to glass and carbon fibers55,56. For centuries, natural fibers used in several applications such as clothing, making baskets, ropes and various parts of automobiles54.
Classification of natural fibers
Natural fibers may be categorized based on their sources57. Natural fibers come from three main sources namely; animal, plant, and mineral materials58. Plant fibers include bast (stem) fibers, seed fibers, leaf fibers, fruit fibers, core fibers, cereal straw fibers, wood fibers and other grass fibers. Animal fibers include hair and wool. Mineral fibers such as asbestos and glass fiber. Figure 6a, b presents the classifications of the different natural fibers and cell wall of natural plants repectively55,57,58,59.
The plant fibers can be categorized into bast fibers, fruit fibers, leaf fibers, grass fibers, seed fibers, wood fibers, and stalk fibers. Bast fibers like ramie, hemp and flax. They come from the skin and bast around the plant stem. Fruit fibers that are collected from fruits such as coconut and palm. Leaf fibers that are extracted from leaves such as sisal, pineapple, abaca and banana. Seed fibers that are developed from seeds and seed shell such as kapok and cotton. Stalk fibers which are generated from the stalks of the plants such as wheat, rice, and corn. Grass fibers include bamboo and bagasse. Finally, wood fiber (hardwood and softwood)58,59,60,61.
Cotton fiber
The purest form of cellulose is cotton. Cotton is hydrophilic. It has high elastic, flexibility and elongation at break. It is degraded by acids but it has resistance to alkalis. Cotton has low microbial resistance however the fibers are highly resistant to beetles and moths. Cotton is burning very fast57. Although cotton fibers are widely used, their mechanical performance suffers behind other natural fibers, notably different types of bast fibers. This can be attributed to the low degree of fiber orientation59.
Coir fiber
Coir is developed from the coconut fruit62. It is found between the outer shell of the coconut and its husk60. Among several natural fibers, coconut fiber has many desirable properties such as durability, high hardness, acoustic resistance, resistance to fungi and moths, and hardly burns. Also, this fiber is more moisture resistant than other natural fibers. It was reported that the coir fiber has saltwater and heat resistance63.
Flax fiber
Flax fiber is the strongest fiber comparing with the other natural fibers64. It is a renewable, and biodegradable material. Great attention increased for flax fiber-reinforced plastic composites because of its great toughness, high strength, stiffness, low density, and bio-degradability. When compared to E-glass fiber, flax fiber has a higher specific tensile strength63. Flax fiber is widely used in several applications such as towels, carpets, ropes and fabrics. Also, flax fiber has been used in biocomposites fabrication65.
Kenaf fiber
The outer (bast) and inner (core) fibers of the kenaf plant are used to produce kenaf fiber57. Kenaf fibers are used in polymer matrix composite as reinforcement materials. It is non-abrasive during manufacturing, has a low density, and good specific properties. It is also biodegradable63. On the other hand kenaf fibers are rough, brittle, and hard to process. It is widely used in textiles, paper materials, ropes, absorbents, and building materials66.
Hemp fiber
Hemp fiber belongs to the bast fiber family63. It is used in polymer composites reinforcement67. It consists of 55-72% cellulose, 8–19% hemicellulose, 2–5% lignin, 4% minerals and less than 1% wax63. It has excellent mechanical strength, elastic modulus and moisture resistance46,52. Hemp fiber has chemical composition comparable to flax fibers, however flax is less resistant to moisture than hemp. Hemp fiber has a toughness that is roughly 20% greater than flax, although they have a poor elongation59. Hemp fiber is used to produce paper, fabrics, construction materials, plastic and composites68.
Jute fiber
Jute fiber is extracted from the ribbon of the stem. Jute fiber has wood-like properties. It has good mechanical and thermal properties, high aspect ratio, high strength to weight ratio and good properties of insulation63. Jute has a microorganism’s resistance, but it is sensitive to photochemical and chemical attacks. Jute fiber is brittle because of high content of lignin and a small extension to break. It has tensile strength lower than flax or hemp. Exposure to sunlight causes loss of the tensile strength of the fiber and has little resistant to acid and moisture. Jute is the plant fiber with the highest hygroscopicity. Jute is silky texture, biodegradable material and resistant to fire and heat which makes it adequate for use in industries. It is used in travel luggage, fashion, furnishings, carpets and other floor coverings. Jute fibers may be used to strengthen polymer composites (thermoset and thermoplastic)69.
Ramie fiber
Ramie fiber is white in color, very fine and silk-like. Ramie fiber is naturally resistant to germs, insects, and mildew. Ramie fiber is not harmed by mild acids and stable in alkaline media57. The physical properties of ramie fiber showed high tenacity, high luster and brightness70. The fiber has an exceptional strength which slightly increases when wet. Ramie fiber is gaining attention as a polymer matrix reinforcement in composites57. Natural fiber has much less density than that of synthetics fibers63.
Sisal fiber
The sisal fiber comes from the sisal plant leaves which is a hard fiber46. As compared to glass fiber, sisal fiber has a higher strength and stiffness71. Sisal fibers include a number of environmentally friendly benefits such as low density, excellent specific modulus, and strength. These advantages make them appropriate for use as reinforcement in composites72. When the temperature of the sisal fiber increased, the tensile strength, modulus, and toughness for fiber decrease. It is smooth, yellow, straight and degraded easily in saltwater56. The fiber increases the toughness of polymer than increasing the modulus and strength. The sisal fiber composites had maximum toughness than other fibers73.
Pineapple leaf fiber (PALF)
PALF is made from the leaves of the pineapple plant51. It’s a cheap waste product from pineapple farming57. PALF consists of high content of cellulose about 70-82%, polysaccharide, and lignin51,57. PALF has superior mechanical properties because of the high content of cellulose53. As compared to cotton, dyes faded more quickly due to the high content of lignin and wax materials in pineapple leaf fiber. Also, dyes difficult to penetrate due to the high coarseness of PALF51.
Abaca
The abaca is a banana species. that comes from the leaf sheaths of the banana plant57,59,63. It is yellowish-white in color and lustrous fiber. It is the strongest fiber among all other plant fibers. Abaca tensile strength is twice times higher than sisal and three times higher than cotton57. It is resistant to degradation in saltwater than other fibers, so it is preferred to use in marine applications63. Abaca fibers are used in many applications such as manufacturing ropes, bags, slippers, placemats and doormats. Also, abaca fiber is used as a reinforcement of composites74.
Bagasse and bamboo
After sugar is extracted from the sugarcane stalk, bagasse is a fibrous residue that remains63. It is used in many applications like biofuel in the production of renewable energy and used as a natural fiber-reinforced polymeric composite63,75. Bamboo fiber is a kind of cellulosic fiber75. It has eco-friendly characteristics such as high growth rate, lightweight, high strength and it is a biodegradable material. It is used in many applications such as flooring to ceiling and transportation, furniture manufacturing, etc. Also, it is used as a reinforcement of composites72.
Palm fiber
It is biodegradable, compostable, and has the potential to break down into its essential parts and return to the environment naturally. Palm fiber has five times the tensile strength of structural steel and is comparable to flax, hemp, and sisal. It has better vibration dampening and acoustic insulation than glass and carbon fiber as well as better heat insulation than carbon60. Palm fiber has a cellulose purity of up to 70% and a thermal stability of up to 226 °C.
Animal fibers
Animal fibers are the most significant natural fibers after plant fibers. They are employed in composites as reinforcement57. Animal fibers include mainly three types: hair, avian and silk fibers. Animal hair fibers are obtained from animals and hairy mammals such as horses, sheep, and goats. Silk fibers are obtained from insects while preparing cocoons or from dried saliva from insects. Avian fibers are produced from bird feathers58.
Structure and composition of natural fibers
Natural fiber cell walls are made up of three layers: a main cell wall, a secondary cell wall, and an intermediate cell wall known as lumen59. Fig. 6b depicts the structural organization of a natural fiber cell wall52,59,76. This structure is known as microfibril63. During cell formation, the wall of the primary cell is the first layer formed. Secondary walls S1, S2, and S3 are the three layers that make up the second cell wall77. The lumen layer is responsible for the transportation of water78. The cell walls are formed of a semi-crystalline cellulose microfibril, hemicellulose, lignin, wax, pectin and water-soluble compounds63,77.
The physical properties of the fiber are connected to the inner structure and components of the plant material that is being used. The fiber of plants is lignocellulosic structures which are consist of hemicelluloses, cellulose, and lignin, as well as pectin, protein, wax, ash, tannins, and inorganic salts59. These components are vary according to the fibers sources, growth conditions, age of plant and processes of digestion59,63. The chemical composition of some of the common natural fibers are presented in Table 159,77,79. Cellulose content of fibers is the most important factor in determining their characteristics and mechanical performance when utilized as reinforcement in composites. In contrast to cellulose, an increase in non-cellulose components causes a decrease in fiber strength and modulus, which has severe consequences for composites reinforced with natural fibers59.
Surface modification of natural fibers
Natural fibers have a variety of drawbacks in reinforcement composites, including poor compatibility with the polymer matrix due to the hydrophobic character of the polymer matrix and the hydrophilic character of the fibers. Their moisture absorption, and dimensional stability are considered as main limitations63,80. The high wettability can be attributed to the existence of OH groups, and polar groups. The mechanical properties of natural fibers decreased when the moisture content in natural fiber increased. This led to a loss in dimensional stability and degradation. This causes weak adhesion between the polymer matrix and the natural fiber when the natural fibers used as reinforcement in composites17. Surface adhesion is a key factor in describing component mechanical and physical characteristics80,81. These problems can be solved by surface modification treatment such as chemical, physical and biological treatments17.
Chemical treatment
Since natural fibers are hydrophilic and polymer matrix is hydrophobic, there is an inherent incompatibility between them, this results at the interface in weak bonding. Chemical treatment methods will reduce the fiber’s hydrophilic nature by removing hydrophilic OH groups from reinforcing fiber81. This treatment strengthens the adhesion of the polymer matrix to the fiber via chemical reactions53,60.
Alkaline treatment or mercerization
This method of treatment is the simplest, most cost-effective, and efficient for improving the adhesive capabilities between polymer matrix and natural fibers53. In this chemical treatment, aqueous sodium hydroxide (NaOH) is used. Natural fibers are soaked in a predetermined concentration of NaOH for a certain time and temperature. Non-cellulosic components like lignin, hemicellulose, oils, and wax are removed during this process60. The following reaction shows alkali treatment in below equation80.
$${{{\mathrm{Fiber}}}} - {{{\mathrm{OH}}}} + {{{\mathrm{NaOH}}}} \to {{{\mathrm{Fiber}}}} - {{{\mathrm{O}}}} - {{{\mathrm{Na}}}}^{{{\mathrm{ + }}}} + {{{\mathrm{H}}}}_{{{\mathrm{2}}}}{{{\mathrm{O}}}}$$
(1)
The removal of non-cellulosic components modifies the polymerization degree and structural orientation of cellulose crystallites, altering the chemical composition of the fibers. Also, this treatment has a permanent effect on mechanical fibers behavior, particularly on their stiffness and strength. Various alkaline treatment studies indicated that mercerization would increase the amorphous cellulose amount at the crystalline cellulose expense and remove hydrogen bonds in the network structure80. Neutralizing the fibers can be achieved by using acetic acid to end the reaction by removing the rest of the hydroxyl groups after washing the fibers with distilled water82.
Silane treatment
Saline is a molecule of multifunctional. It is used as a binding agent to adjust the surface of fibers. Saline coupling agent develops a siloxane bridge chemical bond between the polymer matrix and the fiber. Silanols form when moisture and a hydrolysable alkoxy group are present. One silanol end reacts with the matrix functional group during condensation, whereas the other end reacts with the cellulose hydroxyl group81. The following reactions show silane treatment in below equations83.
$${{{\mathrm{CH}}}}_{{{\mathrm{2}}}}{{{\mathrm{CHSi}}}}\,\left( {{{{\mathrm{OC}}}}_{{{\mathrm{2}}}}{{{\mathrm{H}}}}_{{{\mathrm{5}}}}} \right)_{{{\mathrm{3}}}}\,\mathop{\longrightarrow}\limits^{{{{{\mathrm{H}}}}_{{{\mathrm{2}}}}{{{\mathrm{O}}}}}}\,{{{\mathrm{CH}}}}_{{{\mathrm{2}}}}{{{\mathrm{CHSi}}}}\,\left( {{{{\mathrm{OH}}}}} \right)_{{{\mathrm{3}}}} \,+\, {{{\mathrm{3C}}}}_{{{\mathrm{2}}}}{{{\mathrm{H}}}}_{{{\mathrm{5}}}}{{{\mathrm{OH}}}}$$
(2)
$${{{\mathrm{CH}}}}_{{{\mathrm{2}}}}{{{\mathrm{CHSi}}}}\,\left( {{{{\mathrm{OH}}}}} \right)_{{{\mathrm{3}}}}\,+\, {{{\mathrm{Fiber - OH}}}} \to {{{\mathrm{CH}}}}_{{{\mathrm{2}}}}{{{\mathrm{CHSi}}}}\left( {{{{\mathrm{OH}}}}} \right)_{{{\mathrm{2}}}}{{{\mathrm{O - Fiber + H}}}}_{{{\mathrm{2}}}}{{{\mathrm{O}}}}$$
(3)
The number of hydroxyl groups in natural fibers is decreased during this treatment, which leads to a reduction in absorption of moisture and improves the fiber’s mechanical properties82. The saline treatment increases the composite’s tensile strength while also reducing the influence of moisture on its characteristics. It also enhances fiber-polymer matrix adhesion80.
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Acetylation treatment
In acetylation treatment, acetyl groups are grafted to the cellular structure of the fibers by using acid catalyst84. The fibers are firstly soaked in acetic anhydride. They are processed for 1-3 h in acetic anhydride at an elevated temperature (70–120 °C) in order to accelerate the reaction. This process is known as esterification method for plasticizing natural fibers with the formation of acetic acid as byproduct. The hydrophilic hydroxyl groups of the natural fiber react with the acetyl group (CH3CO). To swell the cell wall and activate the reaction, a small amount of acetic acid is required in the reaction mixture. Equation (4) shows the acetylation treatment of natural fiber80.
$${{{\mathrm{Fiber - OH}}}} + \left( {{{{\mathrm{H}}}}_3{{{\mathrm{C - CO}}}}} \right)_2{{{\mathrm{O}}}}\,\mathop{\rightarrow}\limits^{{{{{\mathrm{CH}}}}_{{{\mathrm{3}}}}{{{\mathrm{COOH}}}}}}_{{{{{\mathrm{conc}}}}{{{\mathrm{.H}}}}_{{{\mathrm{2}}}}{{{\mathrm{SO}}}}_{{{\mathrm{4}}}}}}{{{\mathrm{Fiber - O - CO - CH}}}}_3 + {{{\mathrm{CH}}}}_{{{\mathrm{3}}}}{{{\mathrm{COOH}}}}$$
(4)
When the natural fiber’s moisture content decreases, the fiber’s hydrophilic nature decreases60. The removal of non-cellulosic components from the fiber improves moisture resistance81. The dimensional stability and degradation of composites are also enhanced by this procedure80.
Benzoylation treatment
Benzoyl chloride is utilized in this procedure. This treatment reduces the fiber’s hydrophilic nature, improves fiber-polymer adhesion, and increases composite strength. Alkaline treatment is used while treating benzoylation. Non-cellulose compounds are removed during the alkaline treatment process. The amount of reactive OH groups on the fiber’s surface likewise increases. After that, the fibers are exposed to benzoyl chloride treatment. The hydroxyl groups are replaced by benzoyl groups and found in the cellulose during benzoylation treatment81. This treatment minimizes the absorption of moisture and enhances the thermal stability of the fiber.
Equations (5) and (6) describe the steps of benzoylation treatment85,86.
$${{{\mathrm{Fiber - OH}}}} + {{{\mathrm{NaOH}}}} \to {{{\mathrm{Fiber - O - Na}}}}^ + + {{{\mathrm{H}}}}_2{{{\mathrm{O}}}}$$
(5)
(6)
Peroxide treatment
The connection between the natural fiber and the matrix can be enhanced by peroxide treatment81. Peroxide is a specific functional group with function group RO—OR´ which contains divalent ion O—O. Peroxide tends to decompose to free radical in the form of 2 RO•86. The peroxide decomposition is completely achieved by heating the solution at high temperature81,86. The hydrogen group of the cellulose fiber and the matrix react with the free radical RO•. Benzoyl peroxide and dicumyl peroxide are used in the modification of natural fiber surface. In this treatment, dicumyl peroxide or benzoyl peroxide is used to coat the fibers in acetone solution for about 30 min at a temperature of 70° C81,87 after pre-treatment of alkali. The concentration of peroxide solution is about 6% and saturated peroxide solution is used in acetone86. The following reactions show the peroxide treatment in below equations81.
$${{{\mathrm{RO}}}} - {{{\mathrm{OR}}}} \to 2{{{\mathrm{RO}}}}^ \bullet$$
(7)
$${{{\mathrm{RO}}}} - {{{\mathrm{Cellulose}}}} - {{{\mathrm{H}}}} \to {{{\mathrm{R}}}} - {{{\mathrm{OH}}}} - {{{\mathrm{Cellulose}}}}$$
(8)
where:
R = the functional group of peroxide
Pretreatment on the fiber with benzoyl peroxide or dicumyl peroxide produces high mechanical properties of the composition81. After treatment, the treated fiber should be washed with distilled water87. This treatment increases moisture resistance and improves thermal stability86.
Maleated coupling agents
Efficient interaction between the polymer and the fiber can be achieved by maleated coupling agent81. Maleic anhydride is commonly used to treat the surface of the fibers as well as the polymeric matrix to enhance the compatibility between polymer and natural fiber. This treatment differs from other chemical treatment methods as it is used to treat both the fibers’ surface and the polymeric matrix. The mechanical properties of polymer matrix-natural fiber composites are improved by this treatment60. hydroxyl groups in the amorphous region of the cellulose structure interact with maleic anhydride during grafting, eliminating hydroxyl groups from the fiber. This treatment decreases the hydrophilic tendency53.
Acrylation and acrylonitrile grafting
This treatment is used to improve the bonding between the polymer matrix and the fiber by using acrylic acid (CH2 = CHCOOH). It reacts with hydroxyl groups of the fiber53. The chemical reaction between the carboxylic acids in the coupling agent and the cellulose OH groups form linkage of ester. Equation (9) shows acrylation treatment88.
$${{{\mathrm{Fiber - OH}}}} + {{{\mathrm{CH}}}}_2 = {{{\mathrm{CHCOOH}}}} \to {{{\mathrm{Fiber - O - COCH}}}} = {{{\mathrm{CH}}}}_2 + {{{\mathrm{H}}}}_2{{{\mathrm{O}}}}$$
(9)
This treatment reduces the fiber’s hydrophilic character and increased its moisture resistance. As a result, peroxide radicals are employed to start the grafting of acrylic acid on the matrix. Peroxide’s O—O bonds remove hydrogen atoms from the tertiary carbon of the polymer chain. The stress transfer capacity at the interface between the fiber and the polymer matrix is enhanced by acrylic acid and therefore improves composite properties81. After 1 h of fiber immersion in various concentrations of acrylic acid at elevated temperatures, alkali-pretreated fibers are washed with an aqueous alcohol solution and then oven-dried84,88.
Fiber grafting with acrylonitrile (AN) (CH2 = CH-C ≡ N) is a method for modifying the surface of the fibers. Free radicals are initiated by acrylonitrile and reacted with the molecules of cellulose in fibers and the matrix monomer. The interlocking efficiency at the interface is improved as a result of the copolymerization process between the polymer matrix and the fiber81. It was reported that sisal fibers showed 25% lower tendency to absorb water, high tensile strength, and modulus characteristics of the polymer composite as a result of acrylation treatment84.
Isocyanate treatment
Isocyanate is a coupling agent used to enhance and modify the surface of natural fibers81. In this treatment, the reaction between the functional group of isocyanate (-N = C = O) and the OH groups of cellulose and lignin components of the fibers forms a urethane group81,89,90. Equation (10) shows the isocyanate treatment of natural fiber24.
$${{{\mathrm{Fiber - OH}}}} + {{{\mathrm{R - N}}}} = {{{\mathrm{C}}}} = {{{\mathrm{O}}}} \to {{{\mathrm{Fiber - O - CO - NH - R}}}}$$
(10)
The bonding properties between the polymer matrix and the fibers are improved by forming strong covalent bonds53, resulting in high compatibility with the binder resin in the composites89,90. Also, urea is formed because of the reaction between the moisture content that exists on the surface of the fiber and isocyanate. This urea can react with the celluloses’ OH groups81. The second reaction results in a fiber with a stronger moisture resistance property as well as a good bonding with the polymer matrix. As a result, mechanical properties and compatibility of polymer composites are improved53,81.
Permanganate treatment
The chemical crosslinking at polymer matrix and the natural fibers interfaces can be improved by treating the natural fibers using permanganate53. Permanganate treatment is conducted using potassium permanganate (KMnO4) in a solution of acetone. Reactions between highly reactive permanganate ions (Mn+3) and the OH groups of cellulose are carried out in this treatment. This treatment forms cellulose-manganate which improves thermal stability. The alkaline treatment is required included with this treatment81. Chemical reactions using the permanganate treatment are given in below equations85.
(11)
(12)
Also, the hydrophilic character and water absorption of the fiber-reinforced composites decrease85,86. This procedure improves the adherence of the polymer matrix to the fiber53. A high concentration of KMnO4 (more than 1%) leads to a decrease in the hydrophilic tendency of the fiber91 and causes excess delignification inside the cellulosic structure and reduces the properties of the fibers. According to Sheng et al., reported that treatment of natural fiber using potassium permanganate increased the surface area and made it coarser during the oxidation reaction, which improved mechanical entanglement with the matrix81.
Physical methods
Natural fibers can be physically treated to improve the mechanical interaction between the polymer matrix and the fibers while maintaining their chemical properties60. Physical treatment includes the use of plasma, corona, ultrasound and UV light17. These procedures are only utilized and applied to change the surface qualities of natural fibers53. Plasma treatment is the most common physical treatment because it stimulates the substrate surface when employed in the treatment of natural fibers92,93. Surface contaminants are the cause of changes in surface qualities including dyeability, wettability, and flammability, hence plasma treatment can be employed to reduce surface contamination17. Generally, the hydrophobicity of the natural fiber is increased as a result of plasma treatment16. Plasma treatment would increase surface roughness resulting in good binding to the polymer and the adhesion between the polymer matrix and natural fiber17. Corona treatments can be employed as a stand-alone surface modification or as a pre-treatment to activate cellulose in preparation for future chemical treatments like grafting59,94. Corona treatment improves the compatibility of hydrophobic polymer matrix with hydrophilic fibers.60,95. Ultraviolet treatment is also used in surface fiber modification which leads to an increase in the fiber’s thermal stability96.
Biological treatment
Natural fibers are commonly treated using chemical and physical processes. However, they have some drawbacks such as usage of large volumes of solvents and hazardous chemicals, generation of wastes and pollutants, energy waste, and required costly equipment and chemicals. Microorganisms like fungi, enzymes and bacteria could be used in natural fibers treatment17. Biological treatment is an eco-friendly method97, more economical than other treatments, less energy waste and improves the thermal stability of the fibers. It was also found to be selective in removing pectin and hemicellulose98. Fungal treatment is a new biological treatment that removes of non-cellulose components of the fiber surface by using specific enzymes, increased fineness of the fiber, and fiber individualization can be achieved98,99. Extracellular oxidizing enzymes produced by white-rot fungus react with lignin and aid in its elimination, as well as enhancing hemicellulose solubility and lowering the fiber’s hydrophobicity. Furthermore, fungi generate hyphae, which causes fine pores on the surface of fiber as well as a rough surface to improve fiber/matrix interaction. Without the use of fungus, enzymes might be applied directly to natural fibers. By increasing cellulose content and eliminating lignin and hemicellulose, enzymes like xylanase and laccase increase hardness, tensile, and flexural strength100.
Biodegradable polymers have been designed and developed over the past 20 years. They are used in applications that advantage biodegradation. Biodegradation is a biological process in which bacteria digest dead organic matter and convert it to microbial energy and biological mass while releasing inorganic compounds as by-products. Biodegradation is used in waste management to convert biowaste into compost for soil fertilization through organic recycling. Anaerobic digesters are another type of organic recycling system that produces biogas and digestate, which is subsequently transformed into compost. Biopolymers are developed to be reused in composting facilities and anaerobic digesters with bio-wastes. Biopolymers are also utilized in agricultural plastics that are made to be left in the field and biodegrade after usage. The environmental effect of compounds produced during polymer biodegradation and composting, which might then be dispersed into the environment by compost fertilization or directly diffused during their biodegradation in soil, is a recurring subject. End-of-life possibilities for biodegradable polymers are represented in Fig. 8.
Biodegradation end products
Chemical elements can be found in nature as components of organic molecules (such as polysaccharides, and so on) as well as in inorganic substances (such as NH3, CO2, and so on). Microorganisms transform lifeless organic materials into inorganic chemicals during biodegradation. Glucose molecules, for example, are converted back into the inorganic compounds that plants used to create glucose via aerobic biodegradation. This process is termed as mineralization as it leads in the transformation of organic material molecules into inorganic compounds and minerals. Organic molecules, like natural polymers and certain man-made polymers, are affected by the biodegradation process. Biopolymers are used in the manufacture of plastic materials that are designed to decompose in the soil or compost plants. Biodegradation of a polymeric compounds in which part of the original carbon (Cpolymer) is mineralized (CO2), part is consumed by microorganisms for their own development and reproduction (Cbiomass), and the rest remains as polymeric residue (Cresidue). Other kinds of microorganisms are engaged in the biodegradation process under anaerobic circumstances. As a result, products such as CO2 and CH4 are produced.
Biodegradation during organic recycling
Organic recycling is a treatment process of bio-waste that results in the generation of compost (stabilized organic matter) that is utilized as a soil fertilizer in agriculture. Organic recycling is standardized and industrial biotechnology that involves a multistep biodegradation process in anaerobic digesters or aerobic composting plants. Organic recycling involves three strategies namely; industrial composting, home composting, and anaerobic digestion.
Industrial composting
This technique can be used to handle bio-waste obtained by home, industrial, and agricultural processes. In addition, bio-waste from sewage treatment, park and garden upkeep are used. Composting refers to organic matter recycling method that turns waste into compost. Biodegradable plastic materials that have been used before are an ideal feedstock for industrial composting. Bio-waste is collected in industrial composting plants, where a variety of elements come together to provide a perfect environment for microorganisms to improve the composting process: temperature, moisture, and pH change over time. During the process, O2 should be available. Microorganisms get energy and chemical ingredients for their own survival, development, and reproduction through this mechanism. Microbial metabolism generates heat, which leads to increase pile temperature. As the temperature of the mass increases, quicker reactions take place, speeding up the biodegradation process.
Home composting
When compared to industrial composting, home composting is applied to a lower amount of bio-waste created through domestic activities or garden upkeep and is done in a much more varied manner. As a result, home composting can provide different results due to the fact that numerous elements influence the process such as moisture, temperature and types of microorganisms. Because the compact dimensions of the composting masses may not be allowed for high temperatures to be attained, home composting is frequently slower than industrial composting.
Anaerobic digestion
Bio-waste is decomposed by a bacterial population in the absence of oxygen, resulting in the formation of biogas (methane and carbon dioxide) and digestate, with little or no exothermic heat released. A two-step method is used by most commercial anaerobic digestion systems. Anaerobic fermentation is the initial process, followed by aerobic composting in the second step.
Biodegradation in soil
Several biopolymer-based applications that end up in the soil after use are fast growing in the market. Furthermore, it is used to improve soil quality, mature industrial compost, which is made from a feedstock containing biopolymers, ends up in the soil. Soil is a heterogeneous material governed by a mix of environmental and seasonal elements that tightly control the microbial population’s creation and activity. For example, bacteria colonize an alkaline-neutral humid soil, however fungi require acid dry soil to thrive and operate.
Soil texture and structure
Soil texture is formed based on the percentages of clay, sand, and silt particles that make up the soil. The size of clay particles is less than sand and silt particles. Microbial colony development is influenced by soil texture and structure: sandy, dry, and well-aerated soil. It promotes fungal colonization. Clay compact soil with inadequate aeration supports facultative aerobic or microaerophilic bacterial colonies.
Water content
The texture, structure, and organic material concentration of the soil contribute to the soil’s water retention, which has a big influence on microbial selection. The formation of bacterial colonies is aided by humidity, whereas the growth of fungal colonies is aided by dryness.
Organic matter
Organic matter is the preferred substrate for the formation and expansion of microbial colonies since it is the major fuel source for biodegradation. It also acts as a soil buffer, contributes to soil aeration and humidity, and has a good impact on the preservation of microbial habitat.
pH
Microorganisms display varying degree of sensitivity at different pH values. Therefore, pH changes have a significant impact on microorganisms growth. As a result, soil pH is an important factor in restricting microbial colonization. Bacteria prefer a slightly alkaline neutral pH, whereas fungi prefer an acidic pH.
Temperature
The temperature has an impact on microbial activity and presence because it permits microbe colonies to form. Warm temperature speeds up the chemical processes of microbial metabolism and the enzymatic degradation of polymers.
Oxygen
The amount of oxygen in the soil distinguishes between aerobic and anaerobic biodegradation by stimulating the growth of microorganisms that can live with or without oxygen. The amount of oxygen in the air is usually inversely related to the amount of water and carbon dioxide in the air.
Sunlight
Microbial development on the soil surface is inhibited by ultraviolet radiation. As a result, biodegradation takes place within a few millimeters under the soil surface with the highest concentration in the first 10 cm. Temperature, organic matter content, aeration, moisture, and oxygen need to be adjusted to meet optimal values for microbial activity. Mulch films are agricultural biodegradable plastic films that are not removed after cropping. They remain on or in the soil where they are diced up and mixed with the most biologically active soil layer by milling and ploughing activities promoting material biodegradation97.
Mechanical properties
Natural fibers have good mechanical strength, modulus of elasticity and they are tough. The composites from natural fibers are served for commercial purpose and become a good substitute of synthetic fibers in various applications117. When comparing natural fibers with glass fibers, the most important features of natural fibers are inexpensive, good mechanical properties due to lower density of them, easy processing and handling, renewable resources, recyclable, and has good acoustic and thermal insulation ability118. Kim et al.119 reported that natural fiber-reinforced composites with high-strain rates absorb more energy than glass-fiber-reinforced composites. Natural fibers have drawbacks such as low strength, variation in quality, high absorption of moisture, treatment temperatures are limited, low durability and less resistance to combustion117,118. In general, bast fibers are distinguished for having the better properties for structural applications. Among these, flax fibers give the best possible properties between low cost, lightweight, hardness and high strength. Jute is more common. However, it is not as stiff or strong as flax. Overall, natural fibers are characterized by a low density and renewable materials and can be obtained cheaper than glass fiber. Conversely, the strength of natural fibers is much less than synthetic fibers. In situations where stiffness and weight are crucial, natural fibers outperform synthetic fibers. Natural fibers have various unique features, such as biodegradability, renewability, and eco-friendliness, that make them good candidates for serval applications118. Mechanical and physical properties of some different natural and synthetic fibers are listed in Table ,120,121,122,123,124.
When comparing the mechanical properties of biodegradable polymers and non-biodegradable polymers such as polystyrene (PS), polyethylene (PE), polypropylene (PP), there is a wide values range for the strength of both materials, but conventional polymers are generally stronger than biodegradable polymers. These biodegradable polymers have relatively lower strength than conventional polymers125. Among these biodegradable polymers, PLA is considered as one of the most important polymers due to its good mechanical properties that make them suitable for several applications125,126. The mechanical and physical properties of several biodegradable and non-biodegradable polymers are listed in Table ,127,128,129,130,131.
Natural fibers are used to enhance mechanical properties of biodegradable polymers. Dong et al.132 examined the coir fiber/PLA composite mechanical properties. Figure 10a–d shows that flexural and tensile moduli of PLA biocomposites at the fiber loading from 5 to 20 wt% are moderately improved when compared with neat PLA. It was found that the tensile modulus of untreated coir fiber biocomposites improved to 25.6%, and the flexural modulus was enhanced by 13.4% in the presence of 5 wt% and 20 wt% coir fiber respectively. Increased coir fiber loading to 10 wt% resulted in lower tensile and flexural modulus, which became obvious at 30%. Treated fibers may not improve the modulus of biocomposite particularly at higher fibers loading (20 and 30 wt%). This is because the condition of adhesion between the polymer matrix and the fiber, which has less influence on the tensile modulus of biocomposites than tensile strength.
As shown in Fig. 10a, b, biocomposites have lower flexural and tensile strengths than neat PLA. Biocomposites with treated natural fibers have better strength levels than biocomposites with untreated fibers. Biocomposites have lower elongation at break than PLA at fiber loading 5–20 wt%, as illustrated in Fig. 10c. It was found that the brittleness of PLA composites is higher than that of neat PLA. This is because of the rigid nature of the reinforcement of wet coir fibers with the PLA matrix. When fiber loading reaches 30% in the composite, these composites have a high elongation at break compared to polylactic acid. The increase in elongation at break for PLA biocomposites reinforced with coir fiber up to 30 wt% coir fiber can be attributed to the presence of bundles of excessive non-wet fibers, as shown in Fig. 10d, e. This helps biocomposites deform longitudinally under tensile loading. The alkali treatment of fibers works to remove pectin and lignin from the coir fiber. Thus, increase the cellulose content improves the elasticity and flexibility of coir fibers.
Thermal properties
Thermal analysis is used to examine chemical, physical, and structural response as a result of a temperature change. Temperature is a basic state variable that impacts the majority of chemical processes, physical qualities, and structural changes. Thermal analysis may be defined as any scientific or technical assessment of a material in which temperature is manipulated133. This term, however, has long been restricted to approaches using thermogravimetric and calorimetric effects134. The basic techniques used in thermal analysis are TGA thermogravimetry analysis (TGA), differential thermal analysis (DTA), and differential scanning calorimetry (DSC). The difference in temperature between a sample and a reference is carried out by differential thermal analysis (DTA). The loss of weight measured by thermogravimetry is measured by thermogravimetry analysis (TGA). The determination of heat flow done by differential scanning calorimetry (DSC)53,134. Thermogravimetry analysis is widely used to describe the thermal stability of natural fiber and polymer composites. The thermal stability of natural fiber and polymer biocomposites has been thoroughly characterized using TG/DTG research. The lignocellulosic fibers are affected by temperature134.
Yao et al.135 selected ten natural fibers including wood, bamboo, agricultural residues and bast fibers to study thermal decomposition kinetics. The complete thermogravimetric degradation process of natural fibers at a rate of 2 °C/min is shown in Fig. 11a, b. A clear DTG peak with a noticeable shoulder (arrow) is formed due to cellulose thermal breakdown, which is generally the outcome of hemicellulose thermal decomposition in an inert environment. However, since the shoulder peaks of low-temperature hemicelluloses were overlapping with the cellulose main peaks, they were no longer visible in some situations as shown in Fig. 11b. The high-temperature (tails) is usually presented by the degradation of lignin as proven in Fig. 11a, b. Because natural fibers are lignocellulosic materials, the TG and DTG curves for various are very comparable, indicating that natural fibers have similar thermal breakdown characteristics.
Herrera-Kao et al.136 studied the thermal degradation of three common biodegradable polyesters (polycaprolactone (PCL), polylactic acid (PLA) and polyhydroxybutyrate (PHB)) by using TGA. Degradation curves for each PCL and PLA showed that they have only one decomposition level. Whereas TGA of PHB displayed two degradation. The TGA mass loss curve and the related derivate curve (DTGA) for the examined biopolymers (PHB, PCL, and PLA) are shown in Fig. 11c, d. The maximum rate of breakdown temperature differs according to polymer type. However, both PCL and PLA showed only one phase of mass loss with an initiation temperature higher than 300° C as displayed in Fig. 11c, d. This temperature was 430° C for PCL and 395° C for PLA. At 303° C and 410° C, PHB showed two different deterioration phases, the first stage is the major transition since it accounts for over 90% of the total mass loss.
Dynamic mechanical analysis
Dynamic mechanical analysis (DMA) is used on advanced materials with a rheometer to examine their viscoelastic behavior based on storage modulus (Gʹ), loss modulus (Gʺ) and damping factor (tan Δ). Also, DMA is applied on the advanced composite material to understand the glass transition temperature (Tg)137. Manral et al.138 studied the viscoelastic behavior of neat PLA and natural fibers reinforcing PLA by using DMA at different temperature range 30–100 °C. Samples were exposed to twisting load during DMA test.
Storage modulus (Gʹ)
The storage modulus of a composite material is a measure of storage energy in terms of elasticity when subjected to cyclic load. The quantity of energy absorbed in step with the cycle during oscillatory load on the material is known as Gʹ139. Figure 12a displays the storage modulus fluctuation with relation to temperature at a steady cycle loading frequency of 1 Hz. Results confirmed that polymer reinforced with natural fibers improved the Gʹ or the stiffness of polymer composites. Storage modulus curves showed that flax/PLA has the highest value by obtaining the value of MPa when compared to other composites138. The high Gʹ value indicates that flax/PLA composites are stiffer. When the temperature increases, the value of Gʹ decreases for all composites. As the temperature increased, the storage modulus of the composite decreased due to a decrease in stiffness. The decrease in Gʹ refers to viscous behavior of polymer in addition to low stored energy138.
Loss modulus (Gʺ)
The loss modulus(Gʺ) and storage modulus (Gʹ) are interrelated. When polymer shows viscous behavior, it has the potential to dissipate energy in the form of heat due to intermolecular friction among molecules under loading. Further energy could be dissipated by stiff materials and showing higher Gʺ value. Thus, increasing fiber loading in the presence of biodegrdable polymer such as PLA led to increase in friction force and energy dissipation as shown in Fig.12b 18. Flax fiber reinforcement enhanced the stiffness of neat PLA and resulted in a greater loss modulus. It was reported that flax/PLA composites have higher strength than other natural fiber/PLA biocomposites138.
Damping factor (Tan Δ)
The ratio between the loss modulus and the storage modulus is defined as Tan Δ. The ratio refers to the damping behavior of the substance with different temperatures. Figure 12c shows the damping behavior variation with respect to temperature. Neat PLA reinforced with the fibers reduces its damping behavior. The Tan Δ value of neat PLA is found to be higher than all developed composites. It is found that Jute/PLA has a lower damping factor value (0.65). The glass transition temperature (Tg) of material also determined using damping factor curve where the polymer mobility is about to begin. From Fig. 12c, it was found that the damping curve showed that flax/PLA has the highest value of (Tg) of 68.68 °C compared to other biocomposites. This proves that the flax/PLA requires a higher temperature to start molecules mobility138.
Acoustic properties
Natural fiber-reinforced materials have advantages such as better acoustic properties. Because of the great potential of natural fibers to absorb sound, several studies were reported in the literature showing the possibility of using natural fiber rather than synthetic fiber for sound acoustic absorbers. Natural fibers as sound-absorbing materials have an essential consideration in the fabrication of sound-absorbing materials for buildings in today’s environment. Most natural fibers function as porous absorbers when it comes to sound absorption. Porosity appears to be one of the factors that define the absorption properties of any fibrous material. Also, density and thickness of fiber are other factors. The porous material with sound absorption is characterized by a very weak absorption coefficient in the small frequency range but with better acoustic properties in the high-frequency range118.
Natural fibers have good acoustic properties and are utilized in various applications such as vehicles, soundboards, and thermal and acoustic panels140. Values of acoustic absorption coefficients of selected natural fibers are listed in Table ,142,143,144. Chin et al.145 have made a PLA as a polymer matrix reinforced with kenaf fiber. They found that when the kenaf fiber content is 30%, the maximum sound absorption coefficient 0.987 appears at Hz frequency, and the mechanical properties also improved141.
Surface morphology
Polymers and fiber testing and analysis using surface morphology is one of the areas that need a unique method for creating fully prepared samples. The fibers are tested before and after various processes, including fracture surface. Results are accurate and reliable. Surface analysis can measure dispersed phase at all scales, from micro to nano scale. Scanning electron microscopy provides information on the interaction between matrix and dispersed phase146.
In order to study the PLA and natural fiber biocomposites, Xia et al.147 used alkaline treatment, maleic anhydride (MA) grafting treatment, aminopropyl triethoxysilane treatment, and corona discharge treatment to modify the surface of flax fibers17,148. Lignin, hemicelluloses, waxes, and oils were successfully removed from the fiber surface using an alkali treatment. The fiber surface hydrophobic character was increased. Morphological changes resulting from the various treatment methods are shown in Fig. 13a–e147. It was noticed that the alkaline treatment increased the surface roughness leading to more contact points and an improvement in the mechanical interaction between PLA and fiber as shown in Fig. 13a. Maleic anhydride (MA) and silane act as coupling agents. Figure 13d, e shows the surface of the fibers coated with a layer of MA and silane where increased the interaction between the natural fiber and PLA17,147.
Fracture surface of neat PLA and PLA/flax composites were studied by Xia et al. Figure 14a, b shows that the neat PLA fracture surface is smooth compared with the PLA/flax biocomposites. This refers to brittle fracture. After tensile test, flax fibers were pulled-out from the PLA matrix, leaving a substantial number of fragments147. Also, the remaining fibers showed smooth and clean surface and does not contain any layers of polymer matrix. Furthermore, there are numerous holes in the PLA matrix/fiber interfaces, resulting from a relatively poor interface strength between the untreated fiber and PLA147,148,149. The tough fracture morphology is significantly different from the neat PLA and PLA/untreated fiber composite fracture as shown in Fig. 14c–e. Inside the matrix, fibers are well dispersed, and between the fiber and matrix there are no gaps. The remaining of flax fibers were coated with PLA matrix, indicating that the treated flax fibers and PLA have better interfacial force. The results are in a good agreement with toughness results that are obtained by mechanical testing.
Spectroscopic characterization
The functional groups of polymer composites that might be responsible for the polymer/natural fiber interaction are determined using effective approach of Fourier transform infrared spectroscopy (FTIR)150,151,152,153. The FTIR analysis peaks for various untreated natural fibers are listed in Table 653,154,155,156,157. The spectroscopic properties of treated sisal fibers by benzylated were studied. It was that peaks at cm−1 and cm−1 which are assigned to lignin and hemicellulose. They were successfully removed in the treatment process of sisal fiber. Mofokeng et al.158 studied the interaction of random and oriented PLA/sisal fiber composite using FTIR analysis as shown in Fig. 15. Peaks at – cm−1 are assigned to C = O stretches, – cm−1 corresponding to O–H stretches, and cm−1 referred to C– O–C stretches. Due to the existence of free OH groups in the fiber, the presence of the O–H bonds in the composites became more apparent and wider as the fiber loading increases. They also found that hydroxyl groups developed a new peak at cm−1, which is assigned to the absorbed water molecules53.
Using infrared spectra, Xia et al.145 investigated the untreated flax (UF), alkali-treated fiber, corona-treated fiber, MA grafted fiber, and silane-treated fiber. The samples were analyzed with a spectral resolution of 2 cm−1 and a spectrum of cm−1 to 500 cm−1. The FTIR spectra of treated and untreated flax fibers are shown in Fig. 16. The components of flax fiber are cellulose, hemicelluloses, and lignin. These components can be identified using FTIR spectra. C–O and C–O–C looked stretched at and cm−1, respectively. The –C = O stretching of aliphatic carboxylic acid and ketone groups in the cellulose chain showed the peak at cm−1. The absorption of –C–H in flax fiber cellulose showed a peak at around cm−1. The largest peak was the –OH stretching of D-pyran glucose in the cellulose chain at cm−1. It was noticed that the band at cm−1 (–C = O) stretching of aldehyde carbonyl is assigned to lignin and hemicellulose. Figure 16b shows that the band at cm−1 was vanished in the alkali-treated fiber indicating that the lignin and hemicellulose of flax fiber were removed by NaOH solution treatment. Due to the alkali treatment, the band at cm−1 was not visible in the MA-grafted treated fiber, corona-treated fiber, and silane-treated fiber spectra, as shown in Fig. 16c–e147.
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