Bio-based resins for fiber- reinforced polymer composites
Yongsheng Zhang, Zhongshun Yuan and Chunbao (Charles) Xu Institute for Chemicals and Fuels from Alternative Resources, Western University, London, ON, Canada
During the last century, synthetic resins played an important role as engineering plastics. The interest in developing bioproducts (bio-based fuels, chemicals, and materials) and biorefinery processes has been intensified by the fast depletion of petroleum, new environmental regulations, and the growing awareness of global environmental issues and sustainability [1,2].
Most chemicals and materials (e.g., polymers and plastics) used today are derived from petroleum and at the current rate of consumption, conventional petroleum reserves are projected to run out within the next 50 years . As such, bio-based polymers from renewable resources may overtake the position of the petroleum- based polymers in the market of commodity plastics. The aim of this chapter is to review the work that has been done on utilizing renewable resources for the production of resins or polymers and their applications in composite materials.
Biorefinery is expected to be an important approach for cost-effective manufacture of green chemicals, fuels, and materials. Biomass components (cellulose, hemicellu- lose, and lignin) are natural macromolecules, which can be potentially transformed into useful bioproducts . Additional benefits of using the natural components of biofeedstocks are: it minimizes the steps of reaction and hence waste generation, and the products from biomass have marketing superiority when compared with the conventional petroleum-based products, owing to their natural origin . More importantly, bioresources are abundant and underutilized by far. For instance, among 170 billion metric tons of biomass produced annually by photosynthesis, 75% of them are holocellulose composed of carbohydrates, mainly glucose and xylose. Currently, only 3—4% of carbohydrates are utilized for foods or chemicals. The annual harvest of Canada’s forestry and agricultural sectors is approximately 143 million tonnes carbon, representing an immense carbon source for the production of bio-based chemicals and materials to meet the demand of the society .
Lignocellulosic biomass—composed of three basic components: cellulose (40—80 wt%), hemicellulose (15—30 wt%); and lignin (10—25 wt%) depending on species (as illustrated in Fig. 8.1)—is the most promising candidate for the production of bioproducts due to its wide availability from the agricultural and forestry sectors . Lignin is a polymer of three basic monomers, namely, guaiacyl,
Natural Fiber-Reinforced Biodegradable and Bioresorbable Polymer Composites.
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Figure 8.1 Structures of different biomass fractions (lignocellulose, cellulose, lignin, and hemicellulose).
Source: Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering, George W. Huber, Sara Iborra, Avelino Corma, Chemical Reviews, American Chemical Society.
syringyl, and p-hydroxyphenyl propane . Among its complexity in macromolecular structure, the phenolic and hydroxyl groups in lignin are of particular interest , as they have potential for substitution of expensive petroleum-based aromatics or polyols, such as phenol in phenol-formaldehyde (PF) resins, polyols in polyurethane (PU) resins, and bis phenol-A in epoxy resins, etc.
According to the projections set out in Renewable Vision 2020 , at least 10% of chemicals will be derived from renewable resources by 2020. Bio-based polymers are expected to substitute for a substantial portion of petroleum-based polymers, and new applications of the bio-based thermosetting/thermoplastic materials have been identified, ranging from coatings to plastic industries .
The past decades have witnessed an increasing interest from both academia and industry in biodegradable polymers, wherein biodegradable means hydrolyzable into nontoxic products at temperatures up to 50°C within a year. Many aliphatic polyesters and polyolefins possess such desirable properties, among which polylac- tide (PLA) has proven to be the most attractive and widely used biodegradable polymers.
Composites are materials having two or more distinct phases with a recognized interphase. Usually, two phases are present in a composite: a matrix phase (metal, ceramic, polymer etc.) and a reinforcing phase (fibers or particles) uniformly distributed in the matrix phase. Fibers, from natural or synthetic sources, vary widely in their properties such as strength and flexibility. Common engineering fibers include glass, carbon, and aramid fibers (aromatic polyamide). In a fiber- reinforced composite (FRC), the polymer phase has a low strength and high toughness, while the fiber phase has a large strength but low toughness. In an FRC, stress transfer from one phase to another realizes synergism for the mechanical properties of the composite material.
Fiber-reinforced polymeric composites are of marked industrial significance because of their high specific strength and modulus, as they are often used for structural applications, such as automotive parts, circuit boards, building materials, and specialty sporting goods. Currently, polymers for most composites available on the market are derived from petroleum, while the demand for environmental benign composites is increasing, and many FRC manufacturers are working vigorously to make their products “greener.” Exploring “green” composite materials will contribute to the development of the emerging bioeconomy worldwide. The use of environmentally friendly bio-based polymer matrix has been a natural choice. For example, castor oil was epoxidized to make fiber-reinforced car body panels . There has been much research work on the production of green-composites using soy protein polymers  or modified starch. The composites prepared from the bio-based polymer matrix have found wide application in the construction and transportation sectors.