Handbook of Composites from Renewable Materials, Design and Manufacturing

Preface

The concept of green chemistry and sustainable development policy impose on industry and technology to switch raw material base from the petroleum to renewable resources. Remarkable attention has been paid to the environmental-friendly, green, and sustainable materials for a number of applications during the past few years. Indeed, the rapidly diminishing global petroleum resources, along with awareness of global environmental problems, have promoted the way to switch toward renewable resources-based materials. In this regard, bio based renewable materials can form the basis for a variety of eco-efficient, sustainable products that can capture and compete markets presently dominated by products based solely on petroleum-based raw materials. The nature provides a wide range of raw materials that can be converted into a polymeric matrix/adhesive/reinforcement applicable in composites formulation. Different kinds of polymers (renewable/nonrenewable) and polymer composite materials have been emerging rapidly as the prospective substitute to the ceramic or metal materials, due to their advantages over conventional materials. In brief, polymers are macromolecular groups collectively recognized as polymers due to the presence of repeating blocks of covalently linked atomic arrangement in the formation of these molecules. The repetitive atomic arrangements forming the macromolecules by forming covalent links are the building block or constituent monomers. As the covalent bond formation between monomer units is the essence of polymer formation, polymers are organic or carbon compounds of either biological or synthetic origin. The phenomenon or process of polymerization enables to create diverse forms of macromolecules with varied structural and functional properties and applications. On the other hand, composite materials, or composites, are one of the main improvements in material technology in recent years. In the materials science field, a composite is a multiphase material consisting of two or more physically distinct components, a matrix (or a continuous phase) and at least one dispersed (filler or reinforcement) phase. The dispersed phase, responsible for enhancing one or more properties of matrix, can be categorized according to particle dimensions that comprise platelet, ellipsoids, spheres, and fibers. These particles can be inorganic or organic origin and possess rigid or flexible properties. The most important resources for renewable raw materials originate from nature such as wood, starch, proteins, and oils from plants. Therefore, renewable raw materials lead to the benefit of processing in industries owing to the short period of replenishment cycle resulting in the continuous flow production. Moreover, the production cost can be reduced by using natural raw materials instead of chemical raw materials. The waste and residues from agriculture and industry have also been used as alternative renewable resources for producing energy and raw materials such as chemicals, cellulose, carbon, and silica. For polymer composites applications, an intensifying focus has been directed toward the use of renewable materials. Bio-based polymers are one of the most attractive candidates in renewable raw materials for use as organic reinforcing fillers such as flex, hemp, pine needles, coir, jute, kenaf, sisal, rice xxii Preface husk, ramie, palm, and banana fibers which exhibited excellence enhancement in mechanical and thermal properties. For green polymer composites composed of inorganic reinforcing fillers, renewable resources-based polymers have been used as matrix materials. Significant research efforts all around the globe are continuing to explore and improve the properties of renewable polymers-based materials. Researchers are collectively focusing their efforts to use the inherent advantages of renewable polymers for miscellaneous applications. To ensure a sustainable future, the use of bio-based materials containing a high content of derivatives from renewable biomass is the best solution.

This volume of the Handbook of Composites from Renewable Materials is solely focused on the "Design and Manufacturing" of renewable materials. Some of the important topics include but not limited to: design and manufacturing of high performance green composites; manufacturing of high performance biomass-based polyesters by rheological approach; components design of fibrous composite materials; design and manufacturing of bio-based sandwich structures; design and manufacture of biodegradable products from renewable resources; manufacturing and characterization of quicklime filled metal alloy composites for single row deep groove ball bearing; manufacturing of composites from chicken feathers and poly (vinyl chloride); production of porous carbons from resorcinol-formaldehyde gels: applications; composites using agricultural wastes; manufacturing of rice wastes-based natural fiber polymer composites from thermosetting vs. thermoplastic matrices; thermoplastic polymeric composites; natural fiber reinforced PLA composites; rigid closed-cell PUR foams containing polyols derived from renewable resources; preparation and application of the composite from alginate; recent developments in biocomposites of bombyx mori silk fibroin; design and manufacturing of natural fiber/ synthetic fiber reinforced polymer hybrid composites; natural fiber composite strengthening solution for structural beam component for enhanced flexural strength; high pressure resin transfer molding of epoxy resins from renewable sources; cork based structural composites; the use of wheat straw as an agricultural waste in composites for semi-structural applications and design/ manufacturing of sustainable composites.

Several critical issues and suggestions for future work are comprehensively discussed in this volume with the hope that the book will provide a deep insight into the state-of-art of "Design and Manufacturing" of the renewable composite materials. We would like to thank the Publisher and Martin Scrivener for the invaluable help in the organization of the editing process. Finally, we would like to thank our parents for their continuous encouragement and support.

Vijay Kumar Thakur, Ph.D.

University of Cranfield, U.K.

Manju Kumari Thakur, M.Sc., M.Phil., Ph.D.

Himachal Pradesh University, Shimla, India

Michael R. Kessler, Ph.D., P.E.

Washington State University, U.S.A.

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 colleagues (Bertomeu et al., 2012). Mechanical and thermal properties of soy-based resins were found to be strongly dependent on the hardener employed for cross-linking (Gerbase et al., 2002) and the ratio of resin to hardener (España et al., 2012). Also the reinforcement of soy-based resins with natural fibers was investigated. Åkesson and colleagues found that composites employing flax or hemp fibers yielded good mechanical properties (Åkesson, 2009). Adekunle and others studied the impact behavior and flexural strength of resins reinforced with air-laid and woven flax fibers (Adekunle et al., 2010a). Blends of soy-based resins with other not bio-based resins were e.g. investigated by Zhan and colleagues (Zhan et al., 2008) and by the group around Miyagawa (Miyagawa et al., 2005). Other research groups have dealt with the investigation of modified soybean oil and copolymers (Beach et al., 2013; Li et al., 2010) or the use of epoxidized soybean oil as toughening agent (Xiong et al., 2013).

Probably the feedstock used the second most is flax (linseed oil). Different polymerization techniques including curing have been studied thoroughly (Aust et al., 2012; Chen et al., 2002). Different catalysts and hardeners have been evaluated (Boquillon, 2000). Stemmelen and colleagues succeeded in synthesizing a fully bio-based epoxy resin by curing epoxidized linseed oil with functionalized grapeseed oil (Stemmelen et al., 2011). The group around Kasetait studied the influence of the resin-to-hardener ratio and found a clear dependence of the swelling behavior, thermal and mechanical characteristics as well as biodegradability on the amount of cross-linker (Kasetaite et al., 2014). Miyagawa and colleagues studied properties of resins containing different amounts of epoxidized linseed oil (Miyagawa et al., 2005). Slate fiber reinforcement of flax-based resins was analyzed by Samper and colleagues (Samper et al., 2015).

The epoxidation of camelina oil was investigated by Kim and colleagues (Kim et al., 2015). Parameters such as reaction temperature and time or catalyst ratio were varied to optimize epoxidation. Epoxidized camelina oil was cured using different amounts of hardener (Kasetaite et al., 2014), which influenced thermal, mechanical and swelling characteristics as well as other properties such as biodegradability.

Another feedstock available for the synthesis of epoxy resins is cashew nutshell liquid with its major component cardanol. Jaillet and colleagues investigated the curing of epoxidized cardanol and compared different hardeners (Jaillet et al., 2014). Huo and colleagues studied an epoxy resin based on a mixture of cardanol and lignin (Huo et al., 2014). The research group around Kim investigated the enzymatic epoxidation of cardanol and found that resulting resins yielded high mechanical properties such as a good hardness (Kim et al., 2007). Both homopolymers and copolymers were produced by Rao and Palanisamy (Rao & Palanisamy, 2013). They also investigated the impact of curing temperature and time on the final resin properties. Chemical analysis of epoxidized and phenolated cardanol was carried out by Fouquet and colleagues (Fouquet et al., 2014). Thermal degradation of cardanol-based epoxy resins was analyzed by Shukla and colleagues (Shukla et al., 2015). A blend of epoxidized cardanol and petrochemical resins was produced and its mechanical properties characterized by Unnikrishnan and Thachil (Unnikrishnan & Thachil, 2008).

Sunflower oil was epoxidized by the group around Montero de Espinosa (Montero de Espinosa et al., 2008). Commercial epoxidized sunflower oil was mixed with conventional epoxy resins by Hess and Czub to obtain (partially) bio-based resins with high molar mass (Hess & Czub, 2009). Partially bio-based epoxy resins employing sunflower oil as natural resource have also been produced by the research group around Shaker (Shaker et al., 2008).

Epoxidation of canola oil and subsequent curing employing different amounts of co-curing agents was performed by Kong and colleagues (Kong et al., 2012). The influence of hardener concentration was also investigated (Omonov & Curtis, 2014). Optimization of epoxidation of canola oil by the acid ion exchange resin method was conducted by Mungroo and colleagues (Mungroo et al., 2008).

Corn oil was epoxidized using enzymes by Sun and colleagues (Sun et al., 2011) while the group around Mustata (Mustata et al., 2014) studied the curing behavior of epoxidized methyl esters of corn oil as well as thermal properties oft he obtained resins.

Cottonseed oil was epoxidized by the research group around Dinda (Dinda et al., 2008) and optimized by studying various reactions parameters such as stirring speed or temperature. Alike this group, also Carbonell-Verdu focused on the optimization of the epoxidation of cottonseed oil and also investigated the subsequent curing behavior (Carbonell-Verdu et al., 2015). In particular they studied the effect of mixing different anhydride hardeners at different ratios.

Rapi and colleagues reported the synthesis of an epoxy resin from sugar (D-glucose), emphasizing the good availability and cost-efficiency of their starting products (Rapi et al., 2015). In the same study, also thermal characteristics such as glass transition and thermal degradation were investigated.

Epoxidation of vegetable oil has also been achieved by employing exotic resources:

The research groups around Fache succeeded in obtaining epoxy resins from vanillin (Fache et al., 2015a,b). Chrysanthos and colleagues focused on synthesizing fully bio-based epoxy resins for high performance applications by replacing the diglycidyl ether of bisphenol A (most frequently used monomer for producing epoxy resins) with bio-based isosorbide diglycidyl ethers (Chrysanthos et al., 2011). In addition, a detailed chemical characterization was conducted, curing behavior was studied and thermal/thermo-mechanical properties were analyzed and compared with a conventional epoxy resin. A similar study was conducted by Łukaszczyk and colleagues (Łukaszczyk et al., 2011). After synthesis and curing of epoxy resins based on isosorbide they found good mechanical properties in comparison with a conventional epoxy resin.

The group around Aouf could obtain a partially bio-based epoxy resin by using tara tanning as starting product (Aouf et al., 2014). The cured network resulted in good mechanical and thermal properties.

The research group around Konwar, Das and Kara investigated the exploitation of Mesua Ferrea L. seeds to produce bio-based epoxy resins (Das & Karak, 2010a,b) as well as polyesters and prepared blends of both (Konwar et al., 2011). They conducted extensive characterization of curing behavior and properties of the products and, in addition, produced nanocomposite polymers using clay as nanofiller.

Both Rios and colleagues (Rios et al., 2011) and Goud and colleagues (Goud et al., 2007) engaged the epoxidation of jatropha oil. The former evaluated the utilization of different catalysts, while the latter studied the influence of various process parameters of the epoxidation by the acid ion exchange resin method.

The group around Okieimen conducted a detailed study on the epoxidation kinetics of rubber seed oil and also investigated the effect of temperature on epoxidation (Okieimen et al., 2002).

Manthey et al. performed epoxidation as well as curing kinetics of hemp oil (Manthey et al., 2012).

Olive oil, namely its main component oleic acid, was epoxidized and cured by Nicolau and colleagues (Nicolau et al., 2009). Subsequently, properties of the obtained resin were analyzed by light scattering regarding molecular weight.

Japanese green tea (utilized component: Cammelia sinensis) was employed to synthesize epoxy resins by Basnet and colleagues (Basnet et al., 2015). Thermal and mechanical properties of the resins were assessed and found to be sufficient to possibly replace bisphenol A-based epoxy resins. Benyahya and colleagues conducted a similar study and also found that epoxy resins based on green tea leaves exhibited thermal and mechanical properties similar to conventional epoxy resins (Benyahya et al., 2014).

Gallic acid, a phenolic acid that is present in various plants (for instance in green tea), was epoxidized and cured by Aouf and colleagues (Aouf et al., 2013). Prior to epoxidation they performed allylation of OH groups to gain double bonds.

Wheat straw was used to gain lignin (Biolignin™) and consequently used as a substitute for bisphenol-A in the production of epoxy resins (Delmas et al., 2013). Different resin formulations were characterized and compared regarding thermomechanical properties.

Mahua oil was epoxidized by the group around Goud (Goud et al., 2006). They studied parameters such as resin formulation (proportions of all components and type of catalyst), temperature and stirring speed on the epoxidation.

Crambe oil was utilized for the synthesis of epoxy resins by Raghavachar and colleagues (Raghavachar et al., 1999).

Limonene, a monoterpene present in various plants, was employed by the group around Xu to synthesize partially bio-based epoxy resins (Xu et al., 2004).

All of the epoxy resins described above claim to be bio-resins. However, one must always keep in mind that the term bio does not define the content of resources based renewably. As mentioned above, for the production of epoxy resins, not only the oil (which is usually the part that is bio-based), but also curing agents and often catalysts and other components are employed. Many of the studies dealing with curing of epoxy resins engage in finding the optimal ratio between functionalized oil and hardener. In many cases a ratio of up to 1:1 is found to yield maximum conversion rates and hence best properties of the final product. There are two major substances that are used for the curing of epoxidized plant oils: Cyclic anhydrides and different types of amines. Anhydrides are favored since anhydride-cured resins typically yield high mechanical properties (Ebnesajjad, 2013) owed to the rigid chemical structure of anhydrides as depicted in Figure 1.3. This is of particular importance for plant-based epoxidized oils, since they usually exhibit an aliphatic and hence inherently flexible structure. In addition, exothermicity of the curing reaction is rather low and pot life is long (Campbell, 2010). However, anhydrides are generally not bio-based and are often harmful to both humans and the environment. Furthermore, the employment of catalysts/accelerators is essential (Osamu, 1990). Final resin products are dependent on the type of catalyst, which can be tertiary amines, alcohols and imidazoles. Escpecially imidazole - which is classified as toxic - was found to yield particularly good properties. The mechanism of the anhydride epoxy curing can basically be expressed in two steps. Initially, the catalyst opens the anhydride ring, as depicted in Figure 1.4 (top). Subsequently, the so built ester reacts with the oxirane group of the epoxy, as depicted in Figure 1.4 (bottom). Some side reactions can occur, however, they shall not be discussed here.

Figure 1.4 Schematic anhydride curing mechanism. Exemplary catalyst: alcohol.

Concisely, the problem with anhydride-cured epoxy resins consists of three aspects: First, the small fraction of bio-based starting products; second, the harmfulness of anhydrides; and third, the harmfulness and sometimes toxicity of catalysts.

In place of anhydrides, amines can be engaged as curing agents. They can be classified in cyclic and aliphatic amines. Cyclic amines should be avoided since they are in general either toxic or carcinogenic or in another way harmful. In this regard, aliphatic amines have more advantageous properties. However, as mentioned above, epoxidized plant oils represent a very flexible composition due to the absence of cyclic sections. In order to obtain a sufficient mechanical property portfolio (e.g., high modulus and strength) to allow for application as structural and composite parts, the curing agents needs to compensate for this softness. Yet, aliphatic amines cannot fulfill this requirement since they are flexible, too. Glass transition of such cured resins is accordingly low. Consequently, epoxy resins cured with aliphatic amines are typically only appropriable for applications such as plasticizers. Furthermore, this curing agent is prone to side reactions and often reacts with the ester groups present in the epoxidized oils rather than with the oxirane groups. This also results in a flexible structure of the final resin. Another undesirable characteristic of the curing reaction with aliphatic amines is the pronounced exothermicity. All in all, aliphatic amines are as a rule not a feasible option for preparation of high performance epoxy resins.

One additional option to prepare epoxy resins is a two-step functionalization of the plant oils. For that purpose, the epoxidazion procedure is followed by acrylation. The additional acrylation gives the advantage of new functional groups that allow for alternative cross-linking pathways. The reaction of epoxidized plant oil with acrylic acid is displayed in Figure 1.5.

Figure 1.5 Acrylation of epoxidized plant oil.

The research group around Wool has extensively studied the acrylation of epoxidized plant oil and reported detailed reaction mechanisms (Khot et al., 2001). They analyzed properties of the final resins based on various plant oils (La Scala & Wool, 2005 & 2013; Campanella et al. 2011). Furthermore, this group produced blends and reinforced composites based on acrylated epoxidized plant oils (Beach et al., 2013; O’Donnell et al., 2004; Zhan & Wool, 2010 & 2013). Overall they found that the property portfolio of these novel resins could be accustomed by adjusting the formulation/ratios of substances used for synthesis and concluded that also structural parts can be manufactured using plant oils. However, the acrylation of epoxidized plant oils also has some drawbacks. First of all, resins based on acrylated epoxidized plant oils often have a limited bio-based content. For instance, for one resin the bio-based content was estimated to be around 50% (Lu et al., 2005), a content that was praised to be particularly high. In addition, acrylic acid, which is typically used for acrylation (La Scala & Wool, 2002), is classified as a harmful substance. Worse, acrylated epoxidized plant oil exhibits a viscosity too high for further polymerization. Consequently, viscosity is generally regulated by adding 30–35% of styrene (La Scala & Wool, 2005; Campanella et al., 2011). Styrene, although frequently and in large quantities used in the polymer industry, can cause considerable health problems such as effects on the nervous system and sperm damage, and it is potentially a carcinogen (Agency for Toxic Substances and Disease Registry, 2012). There have been efforts to substitute styrene with other less harmful substances, though, so far no satisfactory solution has been found (Campanella et al., 2011). Furthermore, a catalyst is needed. The catalysts in question, typically peroxides and/or containing heavy metals, usually have very critical properties. Examples are MEKP-based catalysts (Francucci et al., 2012) (MEKP is both toxic and highly explosive), or commercially available catalyst named AMC-2 which contains chromium (La Scala & Wool, 2005; Esen et al., 2007; Bunker & Wool, 2002). Altogether, also the acrylation of epoxidized plant oils is a rather debatable process in terms of safety and environmental friendliness.

An alternative and very interesting approach to cross-link epoxidized plant oils was reported by Tehfe and colleagues (Tehfe et al., 2010). They studied a specific type of photo-curing, the so called free radical promoted cationic polymerization process. The group found out that, by applying silyl radical chemistry, curing of coatings was possible under air and sunlight. Further approaches regarding photo-crosslinking were conducted by the research group around Shibata (Shibata et al., 2009) (bio-based nanocomposite), Decker et al., (Decker et al., 2001) and by Tsujimoto and colleagues (Tsujimoto et al., 2015a,b). The major handicap of conventional photo-induced (radical) polymerization is that it is impeded by the presence of atmospheric oxygen.

1.3 Curing of Bio-Based Epoxy Resins – an Ecological Approach

Next to photoinitiating systems, photo acid generators (PAG) have attracted significant attention for cross-linking various polymeric systems. Traditionally, PAG are deployed in photolithography (Shirai & Tsunooka, 1998; Wolfberger et al., 2015). The mechanism of photo acid generators is rather complex, but many reaction mechanisms can be looked-up in (Shirai & Tsunooka, 1996). In general, PAG are chemical substances which, upon photo cleavage (usually due to irradiation in the wavelength of UV) form Brønsted acids (Shirai & Tsunooka, 1996; Schlögl et al., 2012). A prominent mechanism following this generation of acids is the cationic polymerization of present polymerizable monomers (Steidl et al., 2009). Hence, in the here presented study the potential of curing epoxidized plant oils by employing PAG is evaluated. Two outstanding benefits of cross-linking induced by PAG are that first the cross-linking reactions, once initiated, will continue in the dark and second, reaction is not negatively influenced by the presence of oxygen. Particular focus was on the selection of a PAG both uncritical for humans and uncritical for the environment, which resulted in the choice of N-Hydroxynaphthalimide triflate (Sigma Aldrich, GmbH). One disadvantage that needs to be considered is the high price of this PAG.

The PAG was mixed with epoxidized hemp seed oil. In order to speed up the solution process, tetrahydrofuran was added and subsequently evaporated in a vacuum oven at elevated temperature. Next, PAG decomposition was induced by irradiation with UV light. It was found that final curing of the resin only occurred upon thermal impact. An exemplary curve obtained from differential scanning calorimetry is displayed in Figure 1.6, which clearly shows the exothermal curing reaction upon heating the sample. Just as well, curing can also take place under isothermal conditions (e.g., curing for 1,5 h at 70 °C).

Figure 1.6 DSC spectrum; PAG-initiated thermal curing of epoxidized hemp seed oil.

The conversion of oxirane groups could be confirmed by Raman spectroscopy. Figure 1.7 displays Raman spectra of uncured and cured epoxidized hemp seed oil. Absorption bands at ~1250–1285 cm–1 decrease in intensity, which corresponds to the ring opening of oxirane groups (B & W Tek, Inc; Hardis et al., 2013; Merad et al., 2009). In addition, absorption at ~2900 cm–1 decreases, which might be ascribed to CH-vibrations of the epoxy monomer (Rocks et al., 2004).

Figure 1.7 Raman spectrum; black/full line: uncured epoxidized hemp oil mixed with PAG, grey/dashed line: UV induced thermally cured epoxy resin. Peaks corresponding to the epoxy monomer decrease upon curing.

Altogether, the possibility of curing epoxidized plant oils by the use of PAG could be confirmed. The bio-based content of this bio-resin is particularly high, since the concentration of PAG is only about 0.5%. Currently, further investigation regarding optimization of parameters such as mixing ratios, irradiation time and curing temperature are ongoing. Solution of PAG without addition of tetrahydrofuran is aspired to be absolutely independent of critical substances. Alternative PAG that will be tested include triarylsulfonium hexafluorophosphate salts and diphenyliodonium hexafluorophosphate. These are also comparably uncritical substances and bring economic benefits. In addition, a systematic characterization of the property portfolio of the cured resin including thermal, mechanical, spectroscopic and surface properties is conducted. Ideally, mechanical properties will allow for the application as a matrix in a bio-composite, using only bio-based or recycled reinforcement to gain a maximum ecological product. Yet, realization of such a bio-resin might implicate difficulties in the processing/curing of the resin. UV-irradiation of the epoxidized plant oil could be performed in advance with subsequent positioning of reinforcing materials and final thermal hardening. Of course, several aspects need to be investigated first, such as the stability of the irradiated but uncured resin. In any case, the here presented approach constitutes a promising method for the establishment of a bio-based epoxy resin to be used as matrix for natural fibers.

1.4 Natural Fibers

Natural fibers are classified by their origin, i.e., whether organic (renewable) or inorganic. The former are further subdivided into animal fibers and plant fibers. Very often plant fibers are used when composites based on renewable resources are desired. Kicinska-Jakubowska et al., (2012), and Terzopoulou et al. (2015), present a general overview about several different plant fibers. The hierarchical structure of plant fibers with a more or less pronounced hollow core, so called lumen, and several surrounding walls build-up out of fibrils and microfibrils has been described in several publications (Baley, 2002; Bledzki, 2012; Meshram & Palit, 2013; Sarén, 2006; Terzopoulou et al., 2015). The cell walls consist of three major chemical components: cellulose, hemicellulose and lignin. The highly oriented cellulose microfibrils are embedded in a matrix consisting of hemicellulose and lignin. Different types of plant fibers are characterized by different amounts of the constituent materials. The mechanical performance of plant fibers is defined by the orientation angle, which is measured between the longitudinal direction of the fiber and the longitudinal direction of the microfibrils. Baley, 2002, studied the influence of the fiber diameter and demonstrated the elastic modulus decreases with increasing fiber diameter, and argues this happens due to the presence of the lumen. Since the lumen increase with increasing fiber diameter, the real cross-section of the fiber decreases, this results in an under estimation of the modulus.

A main drawback of natural fibers is the high variation of their properties. Depending on the climate during growth and the harvest period significant different fiber bundle strength and fiber elongation is gained (Graupner et al., 2008; Idler et al., 2011; Svennerstedt, 2009). Furthermore the preserving conditions and different kinds of treatments which might apply (Das et al., 2014) as well as the position along the stem (Lefeuvre et al., 2015) do effect also.

Although natural fibers offer a variety of superior characteristics, e.g., thermal (Medeiros Neira & Santos Marinho, 2009) or acoustical (Bismarck et al., 2006; Liu et al., 2015) ones, most often they are used to fulfil mechanical requirements.

1.4.1 Mechanical Performance of Bast Fibers

Hemp (Cannabis sativa L.) and kenaf (Hibiscus cannabinus) belong to the bast fibers. Bast fibers are located at the periphery of the stem and for technical applications the long technical fibers (approx. 1 meter) are extracted from the plant by mechanical processes. The elementary fibers of kenaf are between 30 and 60 mm long and have a diameter of approx. 25 μm. For hemp the length is between 15 and 30 mm and the diameter varies between 15 and 25 μm. Single natural fibers consist of elementary fiber bundles with about 10 to 40 elementary fibers in cross-section (Figure 1.8).

Figure 1.8 Fiber bundles of hemp (a) and kenaf (b) fibers observed using SEM.

Due to the hierarchical build-up of natural fibers local imperfections in the fibers result in high scatter of mechanical properties. Furthermore, in contrast to synthetic fibers, natural fibers have variable dimensions over the fiber length. Consequently, the measured strength depends on the sample dimensions. The longer the fibers, the higher the probability to have a failure-relevant imperfection in the fiber. To get valid strength data, a systematic statistical approach is required. Using Weibull statistics, the strength can be measured on different and easy to handle test sample length and then extrapolated to requested fiber length (Bos et al., 2002; Acha et al., 2005; Biagiotti et al., 2004; Schledjewski, 2006). Medina, (2007), studied hemp and kenaf fibers: Using a quite simple procedure according to ASTM D 3379-75 the natural fibers are first glued onto a paper frame specifying the aimed gauge length. 20, 40 and 60 mm gauge length are used here and approximately 50 fibers are tested for each length. Force elongation curves are recorded while doing testing on an Instron 4505 Universal Testing machine with a 1N load cell. Before testing the fiber diameter is analyzed at three different positions over the fiber length using optical micrographs received from a light microscope and the program IMAGE C by IMTRONIC GmbH. A two parameter Weibull approach (Weibull, 1951) adapted to account fiber length dependency through a weakest link approximation was used (Paiva et al., 2007; Stoner et al., 1994). The resulting length depending theoretical strength values are summarized in Figure 1.9.

Figure 1.9 Fiber length dependent tensile strength of hemp and kenaf fibers (according to Medina 2007).

For short fiber length the specific fiber strength (tensile strength divided by the fiber density) reaches very good values and especially hemp fibers deliver mechanical performance only slightly below those of glass fibers. To utilize this high performance of the fibers in a composite material a very good bonding between fiber and matrix is required to ensure high load transfer from the matrix into the fiber. Depending on the type of matrix material, i.e., thermoset or thermoplastic, bonding mechanisms are different. Puglia (2005) summarizes different possibilities to enhance the bonding. Also Bledzki & Gassan, (1999), discuss different physical and chemical treatment methods to modify the fiber surface. A lot of effort has been spent to study the affecting mechanisms and how to model and simulate the bonding effects (Guessasma et al., 2009 & 2010; Rjafiallah et al., 2009). Although very promising improvement can be reached by fiber surface treatment, especially in case of chemical treatment, the green character of such methods must be judged very carefully.

Best performance of a fiber-reinforced composite material is reached if uni-directional oriented fibers are used and load direction is identical to fiber orientation. In case of natural fibers fully aligned fibers are hardly reachable. At least a slight waviness of the fibers remains. Ren et al., (2010), studied the effect of fiber orientation angles and fluctuation. They elaborated the fluctuation in fiber orientation to affect the strength more pronounced than the fiber orientation itself. Piyatuchsananon et al., (2015), confirmed these results. Using yarns allows reaching at least pronounced fiber orientation. Madsen et al., (2007a,b), investigated hemp yarn in conventional petrochemical-based thermoplastic resin systems. They conclude, if high stiffness and low weight are desired, hemp yarn reinforcement can substitute for glass fibers as reinforcement in composites. The effect of oriented fibers has been demonstrated also by Pohl et al., (2011). Embedded in a fully biobased furan resin a flax biaxial non-crimp fabric based on an 110tex twisted yarn was used. Resulting specific mechanical properties are mostly directly comparable to those received for glass fiber reinforced material. Only tensile strength tested for 0°/90° reinforcement are lower for the flax reinforcement. But, due to the use of twisted yarns (340 turns m–1 in the present case), the full load capability of the flax fibers has not been utilized here. Gu & Miao, (2014), have analyzed the twisting effect and explain how to reduce the misalignment due to yarn twist. Their proposal for yarn manufacturers is first to spin single yarns at the lowest possible twist level, and then to produce a two-ply yarn from the previous ones at the optimum twist ratio of 0.28.

1.5 Processing Routes

Generally green composites can be processed using the same processing routes as usual for all polymeric-based composite materials. Figure 1.10 depicts an overview about different processing routes for thermoplastic based composites, Figure 1.11 those for thermoset-based composites.

Figure 1.10 Processing routes for thermoplastic based composites (full lines: material use/dashed lines: possible process combinations).

Figure 1.11 Processing routes for thermoset based composites.

For all processes the availability of according resin and reinforcement systems is required. Since most often green composites are mainly used due to low costs, those processing techniques allowing processing the material with short cycle times are preferred. The most common processes applied are compression molding and form-pressing for thermoset-based systems. For thermoplastic-based systems compression molding and injection molding are the preferred ones.

For all processing routes the hydrophilic character of the material, especially the reinforcement, challenges the manufacturing conditions. In most cases conditioning of the material before processing is required to reduce negative effects. In case of resin systems where polycondensation takes place, the condensation residuals also have to be evaporated; otherwise defects like high void content or even pronounced bubbles might occur (Figure 1.12). Therefore, during pressing processes several ventilation cycles, i.e., short unloading and opening of the mold, are often used.

Figure 1.12 Flax random mat reinforced furan resin. In the marked area pronounced bubbles are visible which are a result of the condensation reaction of the furan resin system.

The main limitation during processing is the thermal stability of the materials used. Due to relatively low degradation temperatures of renewable materials heating conditions during processing must carefully be chosen. Especially for thermoplastic-based systems typically two-step processing routes are most common and here the material has to be heated several times. There are only limited possibilities to enhance the processibility. Bodros et al., (2007), propose to use a film stacking procedure, i.e., polymer based films are stacked together with textile reinforcement sheets, for thermoplastic-based materials to have only one temperature cycle. But, due to the significantly extended total cycle time required for such processes it is questionable whether an economical competitive processing route is defined this way. A much more promising solution can be the use of an inert atmosphere, for example by using a vacuum chamber during press processes (Medina, 2009). The vacuum applied will not only prevent any oxidation effects. The vacuum chamber covers the whole pressing area of the press. Since the vacuum is applied around the forming tool, effective ventilation is possible when the mold is slightly opened. This ensures the direct evacuation of any evaporation and, in case of reactive thermoset resin, residual particles of the reaction. This results (Figure 1.13) in reduced odor levels, and reducing cycle times significantly is also possible (Medina, 2009). The vacuum chamber might also reduce the bubble formation as shown in Figure 1.12.

Figure 1.13 Effect of vacuum during form-pressing (according to Medina, 2009).

In case of pressing processes a wide range of pressure can be applied. Typically low pressure processes, i.e., in the range of 2 MPa, are used for processing of natural fiber reinforced polymers. Increasing the pressure will compact the material and void content can be reduced. But due to the specific character of natural fibers too high compaction pressure will result in damage of the fibers. Fibers having pronounced lumen sections can be compressed until the lumen structure is retained and this will increase the mechanical performance. As soon as the lumen structure is damaged, the mechanical performance of the material decreases. This was shown for mixed hemp and kenaf reinforcement by (Medina, 2009).

1.6 Applications and Requirements

Depending on the specific application the requirements a component has to fulfill might be completely different. If several different materials are taken into account, the major question is, Which material will fit best? Green composites do not have any special requirements regarding component design. Although mainly used for non-structural applications (Ashori, 2008), semi-structural or even structural applications are possible. Mansor et al., (2013 & 2014), describes the use of kenaf fiber polymer composites for automotive parking brake lever application. Davoodi et al., (2010 & 2011), presents hybrid bio-composite material, i.e., natural fibers combined with other reinforcements, for bumper beam application. Dweib et al., (2004), discussed the use of all natural composite sandwich beams for structural applications in roof systems for housing applications. In an ongoing project the authors of this contribution are currently working on development of a fully hemp-based composite to be used in highly mechanically loaded components. As a demonstrating component a blade for small or medium sized wind turbine systems is intended. Epoxidized hempseed oil is used for a fully bio-based matrix system and hemp fiber yarns are used as reinforcing material. As usual for such blades, an infusion process will be used (Figure 1.14).

Figure 1.14 Concept of the project Green Composites for Green Technologies.

1.7 Concluding Remarks

There are a lot of advantages being a driving force to further utilize green composites. But, there are also some drawbacks. For example, when selecting natural fiber reinforced composite materials for an application a special focus on water absorption and related effects on mechanical properties is necessary (Dhakal et al., 2007). Tensile and flexural properties decrease and moisture-induced degradation can be observed. These effects are even pronounced if elevated temperatures are used in service.

Furthermore, if renewable materials will be increasingly used, e.g., Joshi et al., (2004), also claims, increased fertilizer use in plant cultivation may result in higher nitrate and phosphate emissions. Negative effects due to eutrophication in local water bodies might happen. A careful dealing with such topics is required.

Mostly conventional composites based on synthetic materials have been used in the past. Increasingly they are now replaced by composites partly based on renewable resources or even fully green composites. Today, application of green composites is already found in several different areas such as automotive and construction. In the future we can expect increased use of such materials.

Acknowledgement

Hemp seed oil was generously given by Waldland Naturstoffe GmbH and epoxidized by Kompetenzzentrum Holz GmbH. Furan resin was generously given by Transfurans Composites. Many thanks for numerous inspirations and assistance in sample preparation go to Simone Radl (Chemistry of Polymeric Materials, Montanuniversität Leoben, AUT). Also Thomas Schmid (Institute of Analytical Chemistry, Johannes Kepler University Linz, AUT) is greatly acknowledged for technical support. Characterization experiments on hemp and kenaf fibers were elaborated by Luisa Medina (Institut für Verbundwerkstoffe GmbH, Technical University of Kaiserslautern, GER) which is greatfully acknowledged. A major part of the work presented was financially supported by the Austrian Ministry for Transport, Innovation and Technology in frame of the program Produktion der Zukunft under contract no. 848668, project Green Composites for Green Technologies.

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