Chapter 1
Design and Manufacturing of High-Performance Green Composites Based on Renewable Materials

Katharina Resch-Fauster1, Andrea Klein1, Silvia Lloret Pertegás2 and Ralf Schledjewski2*

1Chair of Materials Science and Testing of Polymers,

2Chair Processing of Composites, Department Polymer Engineering and Science, Montanuniversität Leoben, Leoben, Austria

*Corresponding author: Ralf.Schledjewski@unileoben.ac.at

Abstract

Fiber reinforced polymers offer high mechanical performance in combination with low weight. Besides conventional composites such as glass fiber reinforced petrochemical-based polymers, new concepts fully based on renewable materials are getting more and more attention. The present chapter delivers an overview about bio-based epoxy resin systems, discusses the challenge regarding curing and proposes an ecological approach regarding curing of bio-based epoxy resin systems. Furthermore, reachable mechanical performance of some bast fiber types is presented in more detail, effects of different processing routes are summarized, and high performance components based on renewable materials are discussed.

Keywords: Natural fibers, bio-resin, epoxidized hemp oil, resin curing, fiber strength, composite processing

1.1 Introduction

Already from the beginning mankind has learned to use materials delivered by nature. Combining materials to reach unique properties is something well known for a very long time. In modern times the knowledge about composite materials, how to select the right constituents and how to combine them to reach superior component properties is well developed. Bledzki et al., (2012), presents a good overview about the history of biocomposites. An early example is linoleum, a mixture of linseed oil, powdered cork and a natural fiber based backing. After more than 150 years linoleum is still a very important and widely used material (Schulte & Schneider, 1996). In the nineteenth and early twentieth centuries many different types of composites based on renewable sources have been developed and used in many different fields of application. One very important area is automotive applications. In 1941 Henry Ford demonstrated the mechanical performance of a rear deck lid by trying to crack it with a sledge hammer. This deck lid was made of paper and soybean resin. Based on the knowledge gained by using natural materials, synthetic materials have been developed and used more and more often. Polymeric matrix systems based on petrochemicals and reinforcing materials like glass fibers and carbon fibers are predominately used for composite materials today. In recent years, materials based on renewable sources, sometimes also called green composites, are getting more and more attention (Evans et al., 2002, Gurunathan et al., 2015; Koronis et al., 2013). Comprehensive reviews concerning biocomposites with a focus on lignin-based types have been published by the group around Thakur (Thakur & Thakur 2014; Thakur et al. 2014a,b).

Materials based on renewable resources do have several advantages. They are available all over the world. At the nova-Institut in Germany basic data is available (Raschka & Carus, 2012). Today, data collected for 2008, only 100*106 ha of the 13.4*109 ha total land area, i.e., less than 1%, is used for the production of renewable resources for material use. "Material use" means, the biomass serves as raw material for the production of all kinds of goods as well as their direct use in products, and excludes the use of biomass where it serves purely as energy sources (Carus et al., 2010). In total 1.65*109 tonnes of biomass have been used in 2008 of which 26*106 tonnes are natural fibers and 24*106 tonnes are plant oils. Approximately 14% of these natural fibers, i.e., 3.6*106 tonnes, are flax, hemp, jute, kenaf, sisal, and related fibers (Raschka & Carus, 2012). All these fibers are plant fibers (Figure 1.1), mainly bast fibers, only sisal is a leaf fiber.

Figure 1.1 Opposite to mainly uniform synthetic fibers, natural fibers are non-uniform and there final shape depends on treatment methods they have been applied to; depicted here are hemp fibers.

Joshi et al. (2004) summarized the reasons why natural fiber composites are environmentally superior as compared to glass fiber composites:

• Lower environmental impact during fiber production
• Typically higher fiber content if natural fibers are used (to reach comparable performance)
• Lower density results in better light-weight performance and reduces fuel consumption and emissions, for example, in automotive applications
• End of life incineration results in carbon credits and recovered energy

Although green composites cover a wide range of different materials, e.g. starch-based resins with feather-based reinforcement (Flores-Hernández et al., 2014) or spent coffee ground powder as reinforcement (García-García et al., 2015), this contribution is focusing on plant fiber reinforced composites. Bio-based epoxy matrix systems are of special interest.

1.2 Bio-Based Epoxy Matrix - State-of-the-Art

In today's society, demand for environmentally-friendly yet well-performing products (and hence materials) is growing vigorously and consistently. Next to the employment of natural fibers, the production of resins based on renewable resources (plant oil) increasingly becomes the center of attention for researchers as well as (composite) manufacturing companies. Plant oils can be gained from numerous different origins, such as a wide range of cereal grains or seeds (Ebnesajjad, 2013). Typically the derived oil is a triglyceride, which means that it consists of glycerol combined with three fatty acids. The structure of the fatty acids is different from crop to crop and defines the property portfolio of the plant oil (Meier et al., 2007). In order to transfer the triglyceride to a polymerizable/hardenable substance, the fatty acids are functionalized. Since epoxy resins are well established in the electronics, aerospace and marine industries, among others (Vaskova et al., 2011), the most commonly used way of functionalization is the epoxidation of fatty acids. Generally, epoxidation of fatty acids requires the presence of double bonds in the plant oil. There are various methods of epoxidation; however, the most (industrially) frequently used option is the so-called conventional method (Baumann et al., 1988). Simplified, a carboxylic acid reacts with hydrogen peroxide in situ, resulting in the formation of peracids. These, in turn, react with the double bonds provided in the plant oil. If performic acid is employed as carboxylic acid, which is often the case, this method is called "in situ performic acid process." The reaction mechanism is schematically displayed in Figure 1.2.

Figure 1.2 Epoxidation scheme of plant oils via in situ performic acid process.

All other epoxidation mechanisms are based on the conventional method and are currently not of major industrial interest. Yet, some of them shall be quoted (Tayde et al., 2011): The "Acid Ion Exchange Resin Method" uses a polymeric catalyst of porous character. Again, hydrogen peroxide and some carboxylic acid act as initial products. The resulting peroxy acid penetrates the catalyst and subsequently reacts with the plant oil in a gentle way. Another approach (the so called "Metal Catalyst Method") represents the substitution of the polymeric catalyst by a metal catalyst in order to increase reaction efficiency and oxirane content. However, it was found that this objective could not always be achieved (Benaniba et al., 2007). So as to increase sustainability and environmentally-friendliness, the catalyst can also be constituted by enzymes ("Enzymatic Method") with the drawback that they are often environmentally-sensitive and hence implying that, for example, temperature needs to be controlled accurately.

Concisely, all major epoxidation mechanisms have a two-step mechanism in common: First, carboxylic acid reacts with hydrogen peroxides forming peracids. This step is typically catalyzed and usually carried out in situ for safety aspects. Second, the peracids react with fatty acids (without catalyst) building oxirane groups.

For the curing of the resin, the epoxidized plant oil is mixed with a hardener (different types of hardening agents will be discussed later on in this chapter), whereat a specific (often stoichiometric) mixing ratio needs to be maintained to ensure optimum hardening. Often, also a catalyst/accelerator is necessary to allow for a reaction of resin and hardening agent. The pathway from the raw material to the final epoxy resin is schematically shown in Figure 1.3.

Figure 1.3 Schematic pathway from raw material to epoxy resin.

Many researchers have succeeded in synthesizing bio-based resins by epoxidizing a large number of different crops. The most exploited feedstock is soy (soybean oil).

Synthesis and processing of epoxidized soybean oil has been studied in detail by the group around (Akesson, 2009) but also others (Adekunle et al., 2010b; Tan et al., 2013; Lu & Wool, 2007). Curing of commercial soy-based epoxy oil has been analyzed by Bertomeu and...