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Natural fiber-reinforced polymer-based composites

Alan Kin-tak Lau1 and Karen Hoi Yan Cheung2

  • 1Swinburne University of Technology, Hawthorn, Melbourne, VIC, Australia,
  • 2Hong Kong Green Building Council, Kowloon, Hong Kong, SAR China


Over the past few decades, research and engineering interest has been shifting from traditional monolithic materials to fiber-reinforced polymer-based composites due to their unique advantages of high strength to weight ratio, noncorrosive property, and high fracture toughness. These composite materials consisting of high strength fibers, such as carbon, glass, and aramid, and low strength polymeric matrix, have now dominated the aerospace, leisure, automotive, construction and sporting industries. Unfortunately, these fibers have serious drawbacks such as (1) nonrenewable; (2) nonrecyclable; (3) high energy consumption in the manufacturing process; (4) health risk when inhaled; and (5) nonbiodegradable. Biodegradation is the chemical breakdown of materials by the action of living organisms which leads to a change in physical, mechanical, and chemical properties. It is a concept of vast scope, ranging from the decomposition of environmental wastes involving microorganisms to host-induced biomaterials.

Although glass fiber-reinforced polymer composites have been widely used due to their advantages of low cost and moderate strength for many years to provide solutions to many structural problems, the use of these materials, in turn would induce a serious environmental problem that is now of concern in most Western countries. Recently, due to a strong emphasis on environmental awareness worldwide, it has brought much attention in the development of recyclable and environmentally sustainable composite materials. Environmental legislation as well as consumer demand in many countries are increasing the pressure on manufacturers of materials and end- products to consider the environmental impact at all stages of their life cycle, including recycling and ultimate disposal. In the United State, it encourages manufacturers to produce materials and products by practicing the 4Rs, which are (1) Reduce the amount and toxicity of trash to be discard (sourced reduction); (2) Reuse containers and products; (3) Repair what is broken; and (4) Recycle as much as possible, which includes buying products with recycled content. After these processes are gone, the materials finally are entitled to be disposed into landfill.

Natural Fiber-Reinforced Biodegradable and Bioresorbable Polymer Composites. DOI:

© 2017 Elsevier Ltd. All rights reserved.

The most common types of conventional composites are usually composed of epoxy, unsaturated polyester resin, polyurethanes, or phenolic reinforced by glass, carbon, or aramid fibers. These composite structures lead to the problem of conventional removal after the end of their life time, as the components are closely interconnected, relatively stable, and thus difficult to be separated and recycled. The recent development of aircraft, such as Boeing 787 and AIRBUS 350, use over 50% of composites as their structural components. A serious problem that brings a strong debate is on the recyclability of the composites after the end of their service life. During the production process, the use of energy to make fibers and resins is another arguable item. However, the advantage of using these materials is due to their light-in-weight, noncorrosive properties, and the ease of manufacturing in different forms and shapes without involving the use of heavy equipment. Therefore, if the natural fiber and biodegradable matrix are used for a new type of biocomposite and can achieve similar functions and strength as glass fiber- reinforced polymer (GFRP) composites, it would help solve many environmental problems addressed above and help improve the living environment in our planet.

Within the past few years, there has been a dramatic increase in the use of natural fibers, such as leaves from flax, jute, hemp, pineapple, and sisal, for making a new type of environmentally-friendly composite. Recent advances in natural fiber development, genetic engineering, and composite science offer significant opportunities for improved materials from renewable resources with enhanced support for global sustainability. In general, two types of natural fibers are identified for making fiber-reinforced polymer; they are (1) plant-based fibers and (2) animal- based Fibers. For the former, due to their abundant supply in the natural environment, the raw material cost is relatively low and can compete with synthetic fibers, such as glass to make the composites. Animal fibers, however, are difficult to collect from wildlife and, normally, have to be obtained from home-fed animals, such as spiders and cocoons.

By using the plant-based natural fiber as reinforcement of polymer-based materials the reduction of the use of synthetic fibers and undegradable polymer for composite structures can be targeted. Excessive use of petroleum-based plastics induces huge amounts of nondecomposable solid waste which causes a serious depletion of landfill capacities. The awareness of the soaring waste problems on the environment has awakened a new interest in the area of materials science and engineering. Because of the increasing environmental consciousness in the society, it is a critical topic for researchers to study different alternatives to replace nonrenewable materials, especially for petroleum-based plastics. Therefore, different types of fully biodegradable materials have been developed recently, as substitutions for nonbiodegradable petroleum-based plastics, and even metallic components [1—4].

Among all the natural fibers, sisal [5], hemp [5], basalt [6], kenaf [7], flax [8], and bamboo fibers [9] are the most common types to be used due to their abundant supply in the natural environment. However, the skill of how to extract the fibers with consistent physical, material, and mechanical properties is key. Besides the surface treatment and processing temperature of the fibers, also their high moisture absorbability is a factor that makes them difficult to be used in high-end engineering products and structures. In some scenarios, the processing temperature of thermoset or thermoplastics during the injection modeling process may cause the thermal degradation of fibers, which substantially reduces the mechanical strength of the composites.

Vaisanen et al. [10] have addressed the advantages of using organic waste and residues from agricultural and industrial processes to develop a new class of composites, in which some of them are good at a relatively high servicing temperature condition (~300°C). Fig. 1.1 shows the thermal decomposition ranges for natural fiber polymer-based composites at different temperatures. In general, natural fibers are degraded at temperatures starting from 200°C up to 500°C (from hemicelluloses to lignin). In the civil engineering industry, the use of natural fibers to reinforce cementitious materials have become more popular due to the advantages of low cost, low density, moderate strength, and local availability in different countries. However, the moisture absorption problem of plant-based fibers is still a critical issue that affects the resultant strength of the composites [11].

To popularize the usage of plant-based natural fiber in civil infrastructure applications, durability, ultraviolet (UV) degradation, and corrosion resistant and inflammable properties are important [12]. Fire-retardant filler compounds are normally used for plant-based natural fiber-reinforced polymer composites. The criteria of selecting the fillers are low cost, relatively easy addition into the polymer, and high fire resistance. Aluminum trihydroxide (ATH), which is also known as alumina triydrate, is an active fire-retardant filler compound most often used in polymers and polymer composites. This compound is decomposed during the dehydration process, to form carbon-inorganic residues and finally becomes a foam-like structure to isolate heat release. It was proved that using a small amount of fire-retardant compound (ammonium polyphosphate [APP]) with kenaf and hydrophobic plastic (PP) can improve the fire-retardant ability by 200%, with wool it is 250%.

Thermal decomposition ranges for natural fiber polymer composites and subsequent effects on the characteristics of the composite constituent [10] (VOCs = volatile organic compounds)

Figure 1.1 Thermal decomposition ranges for natural fiber polymer composites and subsequent effects on the characteristics of the composite constituent [10] (VOCs = volatile organic compounds).

Animal fibers, due to their high protein-based content, are suitable for biomedical engineering applications, particular for the design of implant structures, like bond fixators. A material that can be used for medical applications must possess a lot of specific characteristics. The most fundamental requirements are related to biocompatibility, i.e., not to have any adverse effect on the host tissues; therefore, those traditional composite structures with nonbiocompatible matrix or reinforcement are substituted by bioengineered composites. Table 1.1 summarizes several important factors that need to be considered in selecting a material for the biomedical

Table 1.1 Key factors for the selection of materials for biomedical applications [2]











1st Level material properties

• Chemical composition (bulk and surface)


  • • Elastic modulus
  • • Shear modulus
  • • Poisson’s ratio
  • • Yield strength
  • • Compressive strength

2nd Level material properties Specific functional requirements (based on applications)

  • • Adhesion
  • • Biofunctionality
  • • Bioinert
  • • Bioactive
  • • Biostability
  • • Biodegradation behavior
  • • Surface topology
  • • Texture
  • • Roughness
  • • Form & geometry
  • • Coefficient of thermal expansion
  • • Electrical conductivity
  • • Color, aesthetics
  • • Refractive index
  • • Opacity or translucency
  • • Hardness
  • • Flexural modulus
  • • Flexural strength
  • • Stiffness or rigidity
  • • Fracture toughness
  • • Fatigue strength
  • • Creep resistance
  • • Friction and wear resistance
  • • Adhesion strength
  • • Impact strength
  • • Proof stress
  • • Abrasion resistance

Processing and fabrication

Reproducibility, quality, sterilizability, packaging, secondary processability

Characteristics of host: tissue, organ, species, age, sex, race, health condition, activity, systemic response

Medical/surgical procedure, period of application/usage Cost

Stress—strain curves for different fibers

Figure 1.2 Stress—strain curves for different fibers.

Source: Guinea GV, Elices M, Perez-Riguerio J, Plaza GR. Structure and properties of spider and silkworm silk for tissue scaffolds. In: Basu, editor. Advanced in Silk Science and Technology. Chapter 10. New York: Elsevier; 2015.

applications [13]. Spider and silkworm silks are identified as good reinforcements for making composites for tissue scaffolds. The mechanical properties of these fibers as compared with Nylon is shown in Fig. 1.2. It is obviously seen that the spider silks (Nephila inaurata and Argiope trifasciata) possess high tensile modulus and strength as compared with Nylon 6.6 and cocoon silks [14,15]. However, the main concern for the use of these silks is the consistency of where the silks have originated.

Bioengineering refers to the application of concepts and methods of the physical sciences and mathematics in an engineering approach towards solving problems in the repair and reconstructions of lost, damaged, or deceased tissues. Any material that is used for this purpose can be regarded as a biomaterial. According to Williams [16], a biomaterial is a material used in implants or medical devices, intended to interact with biological systems. Those common types of medical devices include substitute heart valves and artificial hearts, artificial hip and knee joints, dental implants, internal and external fracture fixators, and skin repair templates etc. One of the major features of composite materials is that they can be tailor-made to meet different applications’ requirements.

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