Fruit waste valorization for biodegradable biocomposite applications: A review

Abstract

Currently, food waste is a major concern for companies, governments, and consumers. One of the largest sources of food waste occurs during industrial processing, where substantial by-products are generated. Fruit processing creates a lot of these by-products, from undesirable or “ugly fruit,” to the skins, seeds, and fleshy parts of the fruits. These by-products compose up to 30% of the initial mass of fruit processed. Millions of tons of fruit wastes are generated globally from spoilage and industrial by-products, so it is essential to find alternative uses for fruit wastes to increase their value. This goal can be accomplished by processing fruit waste into fillers and incorporating them into polymeric materials. This review summarizes recent developments in technologies to incorporate fruit wastes from sources such as grape, apple, olive, banana, coconut, pineapple, and others into polymer matrices to create green composites or films. Various surface treatments of biofillers/fibers are also discussed; these treatments increase the adhesion and applicability of the fillers with various bioplastics. Lastly, a comprehensive review of sustainable and biodegradable biocomposites is presented.

Full Article

Fruit Waste Valorization for Biodegradable Biocomposite Applications: A Review

Alison C. Gowman,^a,b Maisyn C. Picard,^a,b Loong-Tak Lim,^c Manjusri Misra,^a,b,* and Amar K. Mohanty ^a,b,*

Currently, food waste is a major concern for companies, governments, and consumers. One of the largest sources of food waste occurs during industrial processing, where substantial by-products are generated. Fruit processing creates a lot of these by-products, from undesirable or “ugly fruit,” to the skins, seeds, and fleshy parts of the fruits. These by-products compose up to 30% of the initial mass of fruit processed. Millions of tons of fruit wastes are generated globally from spoilage and industrial by-products, so it is essential to find alternative uses for fruit wastes to increase their value. This goal can be accomplished by processing fruit waste into fillers and incorporating them into polymeric materials. This review summarizes recent developments in technologies to incorporate fruit wastes from sources such as grape, apple, olive, banana, coconut, pineapple, and others into polymer matrices to create green composites or films. Various surface treatments of biofillers/fibers are also discussed; these treatments increase the adhesion and applicability of the fillers with various bioplastics. Lastly, a comprehensive review of sustainable and biodegradable biocomposites is presented.

Keywords: Fruit waste; Biodegradable; Biobased plastics; Biocomposites

Contact information: a: School of Engineering, Thornbrough Building, University of Guelph, Guelph, N1G 2W1, ON, Canada; b: Bioproducts Discovery and Development Centre, Department of Plant Agriculture, Crop Science Building, University of Guelph, Guelph, N1G 2W1, ON, Canada; c: Department of Food Science, University of Guelph, Guelph, N1G 2W1, ON, Canada;

* Corresponding authors: mmisra@uoguelph.ca; mohanty@uoguelph.ca

INTRODUCTION

One of the biggest challenges facing the world today is to produce enough food to feed the rapidly growing population. Despite efforts to produce enough food, much of it is wasted before it even reaches the consumers. According to the Food and Agriculture Organization (FAO) of the United Nations, approximately 1.3 billion tonnes of food is lost every year (Gustavsson et al. 2011). Fruit and vegetable losses in industrialized nations can exceed 32% before reaching the point of sale (Gustavsson et al. 2011). A prominent solution to reduce food loss is to find alternative uses for products that are rejected by consumers or leftovers from food processing (ASTM D6400-04 2004). Research relating to food waste has been ongoing since the 1990s (Kroyer 1995). To assess the feasibility and prepare to utilize these by-products in industry, it is essential to identify, quantify, characterize, and analyze the by-products of choice (Rosentrater 2004). Full utilization of the by-products can only be realized when the materials’ information is available.

One way to increase the use of food waste is to process it into filler and then incorporate it into a polymer matrix to provide strength and decrease the amount of polymer required in the composite. A further benefit to this approach is the reduction in material cost, as the more expensive polymer is replaced with low-cost filler. By combining different fruit waste fillers with a polymer, composite systems of unique properties can be tailored for specific applications. Common properties of interest include strength, toughness, and thermal stability, among others. These composites can be classified as biocomposites because they contain a biologically derived resource, i.e., the natural filler. They can be considered biodegradable if a biodegradable polymer matrix is used. In this article the term “biocomposite” will be used when both the polymer matrix and the filler particles both come from plant sources. Materials with a petrochemical-based matrix and any kind of reinforcement, including plant-based, will just be called “composites”.

“Biodegradability” is an all-encompassing term used to describe a material’s ability to break down. However, a subsection within biodegradability is compostability. Compostable materials have the ability to be degraded into benign substances under certain conditions, as defined within internationally recognized standards. Factors that affect the degradability of a sample include its size, composition, and thickness, among other characteristics (ASTM D6400-04 2004). Biodegradable or compostable plastics have less environmental impact compared to non-biodegradable or non-compostable plastics because they do not place such a heavy burden on the environment. The biodegradable, compostable plastics contribute to a circular economy in which the products at the end of their life return to the soil and new starting material begins again. There is an increasing demand for these types of products, as consumers are becoming more aware of the impact of non-biodegradable plastics on the environment. Compostable alternatives can potentially alleviate disposal concerns and help create sustainable, environmentally friendly products.

This work is instrumental in providing information to both consumers and industry on the availability of sustainable polymers and composites. The cost benefits associated with the use of natural fillers and sustainability are among the many benefits to using these materials. This work discusses the availability of fruit waste materials around the world. Often, these materials can be obtained at little to no cost, as the natural fillers generated from fruit waste have limited purpose to date. The circular economy, as discussed in this work, focuses on the regeneration of value-added products from previously used materials (Ellen MacArthur Foundation 2017). A circular approach is far more sustainable in the long run, as compared to the linear practices many companies follow today, which require resources to make products and dispose of products at the end of life. Thus, the use of biocomposites is extremely important going forward, especially in single-use plastic applications (Ellen MacArthur Foundation 2017).

Biopolymers can be classified into different categories, depending on how they are produced, whether or not they are biodegradable, and their structures. There are currently many different types of biopolymers available on the market. The major considerations for classification of biopolymers are the source of the starting material and its biodegradability. Recently there has been research on the combination of petroleum-based or biobased polymers with natural fillers or fibers, which may or may not be biodegradable. It is important to clarify that although a material may be biologically based, it might not be biodegradable. This paper intends to focus on biodegradable polymers, regardless of their starting material.

There have been some studies examining composites derived from natural fillers or fibers and non-biodegradable polymers, which are more sustainable than entirely petro-based materials. These studies feature the combination of fruit waste with petroleum-based polymers such as high density polyethylene (HDPE) (Banat and Fares 2015; Satapathy and Kothapalli 2018), polypropylene (PP) (Naghmouchi et al. 2015; Essabir et al. 2016), and epoxy and polyester resins (Durowaye et al. 2014; Ruggiero et al. 2016).

Research by Banat and Fares (2015) combined olive pomace and HDPE to make composites. The addition of a coupling agent improved the interfacial adhesion such that the olive pomace could act as a reinforcing material. Recycled HDPE (RHDPE) has been used in combination with banana fibers, a compatibilizer, and additional filler to generate composites. Banana fiber was a renewable, cost-effective, and non-abrasive filler used to increase the sustainability of an engineering thermoplastic (Satapathy and Kothapalli 2018). Other petro-based thermoplastics have been used as a matrix material. Polypropylene, for example, was combined with olive stone pomace by Naghmouchi et al. (2015) to develop composites. The researchers added maleic anhydride-grafted PP to improve the interfacial adhesion and further improve the mechanical performance (Naghmouchi et al. 2015). Coir fiber and shell particles were combined with PP by Essabir et al. (2016) to generate composites. Interestingly, the tensile modulus improved with the combination of the fiber and coir particles. The sustainability of the composites was improved, supporting their use with biobased and biodegradable polymers. Furthermore, the addition of coupling agents improved the performance of the fabricated materials (Essabir et al. 2016).

Unsaturated polyester resins (as a matrix material), a catalyst and an accelerator were combined with coconut shell fibers and palm fruit to generate composites. The optimal filler contents for coconut shell and palm fruit filler were 10 wt% and 20 wt%, respectively (Durowaye et al. 2014). Date stones, also known as the seeds, were used as the filler in resins to make composites via casting methods (Ruggiero et al. 2016). The addition of a low-cost filler helps to generate renewable materials for a more sustainable future. The reduction in cost, compared to neat polymer, is also an incentive for use in industrial production of composites. Aht-Ong and Charoenkongthum (2002) combined low-density polyethylene (LDPE), banana starch, and ethylene vinyl acetate copolymer (EVA). The EVA acted as a compatibilizing agent to improve the interactions between the filler and the matrix. These films functioned well, but there could be improvement with the sustainable content overall.

There has been limited research performed with tomato pomace and biobased or biodegradable plastics to date. However, post-harvest tomatoes have been used in combination with ethylene vinyl alcohol (EVOH) to make films (Nisticò et al. 2017). The films reduced the cost and overall consumption of petro-based plastic materials because tomato waste was able to act effectively as an additive. The addition of the natural filler improved the overall sustainability of the materials (Nisticò et al. 2017).

There is a popular misconception that the addition of natural fibers changes the biodegradability of non-biodegradable polymers when used to make composites. However, this is not the case. The addition of natural fillers or fibers increases the sustainability of the samples by increasing the renewable content but does not change the inherent nature of the continuous phase. If the continuous phase is not biodegradable, the addition of natural fibers will not change its biodegradability. Therefore, it is important to note that not all composites containing plant-based material are biodegradable, since their biodegradability is most determined by the matrix material.

Overall, this paper provides a comprehensive review of biodegradable composites containing natural fillers, highlighting the feasibility and importance of natural, renewable, and sustainable materials. Going forward, producers of plastics (especially single-use plastics) should keep in mind the end-of-life plan during product development to ensure a cleaner future for the environment. This ideology is discussed extensively by the Ellen MacArthur Foundation, calling it the “circular economy approach.” The circular economy approach, as shown in Fig. 1, illustrates how materials can be reused and recycled. In this work, the valorization of fruit waste re-uses an existing by-product to generate value-added and novel products (Ellen MacArthur Foundation 2017).

Fig. 1. A circular economy approach to sustainable product development. Reprinted with permission from John Wiley & Sons: Journal of Industrial Ecology, (Zink and Geyer 2017), Copyright 2017, License Number: 4561921510933.

BIODEGRADABLE POLYMERS

Biodegradable polymers can be degraded into water and carbon dioxide in the presence of microorganisms. Their degradation is strongly dependent on their surrounding environmental conditions, such as temperature and humidity (Mohanty et al. 2005). A polymer is deemed biodegradable when the polymer degrades during its application or soon after its application (Göpferich 1996). When a significant loss in properties occurs due to thermal, mechanical, or chemical degradation, or from photoexposure, a polymer can be deemed degradable (Göpferich 1996).

Degradable materials can be further specified as compostable. According to standards such as ASTM D5988-18 (2018) or ASTM D5338-15 (2015), a compostable plastic is a plastic that yields carbon dioxide, water, inorganic compounds, and biomass at a rate comparable to other known compostable materials and leaves no visible, distinguishable, or toxic residue. After 12 weeks there should be 10% of the original dry weight or less remaining when sieved on a 2 mm sieve (ASTM D5338-15 2015; ASTM D5988-18 2018).

Biobased or Renewable Resource-based Biodegradable Polymers

Biobased or renewable resource-based biodegradable polymers are polymers produced from renewable resources. Some renewable resources used to fabricate polymers are plant-based, such as sugarcane or corn. Other renewable sources include bacterial fermentation of sugars. The most common biobased and biodegradable polymers include polylactic acid (PLA), polyhydroxyalkanoates (PHAs), and starch.

Polylactic acid (PLA)

Polylactic acid is a widely used, fully biobased polymer made from cassava, sugarcane, or corn. Polylactic acid is widely used due to its biodegradability, good mechanical properties, and comparatively low cost. There are different types of PLA, depending on the orientation of its monomer units. Lactic acid has two chiral carbons, which allow it to have different stereoisomers, namely L-lactide, D-lactide, and L-D-lactide. The properties of PLA are highly dependent on the stereochemical composition. When lactic acid (the precursor to PLA) is derived from biological sources, the majority of it is L-lactic acid, with a small amount being D-lactic acid. Therefore, PLA produced from biological sources is mainly composed of poly(L-lactide) (Garlotta 2001).

Polylactic acid is also a versatile material which can be processed in many ways, such as injection molding, thermoforming, blow molding, extrusion blown film, foaming, sheet extrusion, and film extrusion (Auras et al. 2010). The main disadvantages associated with PLA are its poor impact strength and low heat distortion temperature. Due to these disadvantages, many studies have been performed to overcome these properties by melt blending PLA with other polymers or additives to improve its properties (Mekonnen et al. 2013; Nagarajan et al. 2016).

Polyhydroxyalkanoates (PHAs)

Polyhydroxyalkanoates are polyesters created from bacterial fermentation. They can be synthesized by different strains of bacteria with each synthesized material exhibiting unique properties. The composition of the derived PHA polymer depends on the growth medium used (Reddy et al. 2003).

Polyhydroxyalkanoates can be classified into different groups, depending on their carbon numbers. Short chain length (SCL) PHAs have monomers with 2 carbon atoms to 5 carbon atoms, whereas medium chain length (MCL) polymers have monomers with 6 carbon atoms to 14 carbon atoms (Park et al. 2012). While there are many types of PHAs that can be synthesized, most are not commercially available. The most common industrially produced PHAs can be listed as medium chain length PHA (MCL PHA), polyhydroxybutyrate (PHB), and poly(hydroxybutyrate-co-valerate) (PHBV) (Chen 2009).

Polyhydroxybutyrate has poor melt processability and thermal stability, crystallizes slowly, and is brittle (Muthuraj et al. 2018). The properties of PHB can be modified and improved by copolymerization with hydroxyvalerate (HV), resulting in a product called PHBV, which, with its high HV content, has better ductility and toughness compared to PHB (Avella et al. 2000). Although PHBV is biocompatible, it has poor thermal stability and is expensive, difficult to process, and brittle. The crystallinity and polymer type of PHAs determine their degradation rate. The main uses for PHAs are plastic shopping bags, medical applications (such as tissue scaffolding and bone healing), agricultural films, fishing nets, and cosmetic packaging (Philip et al. 2007). Other researchers have reviewed many different articles on the preparation, properties, potential applications, and future development of PHAs (Avella et al. 2000; Philip et al. 2007).

Starch

Starch is a carbohydrate polymer that consists of many glucose units joined together by glycosidic bonds. It contains approximately 20% to 25% linear amylose and 75% to 80% helical/branched amylopectin by weight. It is found in many plants, such as potatoes, cereal grains, rice, maize, and cassava, among others. The characteristics of starches can differ depending on the amylose-to-amylopectin ratio, crystallinity, and glass transition temperature (T_g). The properties associated with starch, such as the high T_g (approximately 240 °C), strong inter- and intra-molecular hydrogen bonding, water sensitivity, and poor flowability of the starch granules, can limit processability and therefore reduce the amount of applications starch can be used in (Mekonnen et al. 2013).

These challenges can be overcome by the incorporation of plasticizers, blending with polymers, chemical modification, or a combination of these methods (Mekonnen et al. 2013). Starch can be plasticized with water, glycerol, glycol, sorbitol, fructose, mannose, fatty acids, etc. (Rosa et al. 2009; Luchese et al. 2018). Chemical modifications such as hydroxylation, acylation, oxidation, and acetylation have been reported to improve its processability and mechanical properties (Zamudio-Flores et al. 2010; Mekonnen et al. 2013). Although chemical modification is an effective way to modify starch, it can produce harmful by-products and is normally quite expensive. Therefore, plasticization is thought to be the best method to enhance the properties of starch.

Biobased and Organic Additives to Biodegradable Polymers

Biobased additives are derived from natural sources and are added to composites or films to improve their properties or material stability. Most often, the additives are used as plasticizers or binding agents.

Glycerol

Glycerol, also known as 1,2,3-propanetriol, is a simple polyol containing three hydroxyl groups. Animals, plants, and microorganisms naturally synthesize glycerol. Industrially, it can be produced from biobased or petro-based sources (Pagliaro et al. 2007). Here, the synthesis of biobased glycerol is the focus. Biobased glycerol can be obtained from saponification or from the hydrolysis of fats and oils. Saponification functions by creating soap from the fermentation of sugars to glycerol by microorganisms. It can also be obtained from transesterification of fats and oils in biodiesel production as a coproduct (Pagliaro et al. 2007). Glycerol is often used as a plasticizing agent when combined with other materials such as starch (Park et al. 2009; Deng and Zhao 2011).

Sorbitol

Sorbitol is a natural alcohol found in fruits and is a common plasticizer in biobased/edible films to aid extensibility and permeability (Shaw et al. 2002). It is the high molecular weight molecules that maintain a solid state a room temperature. When combined with film materials, it can impinge on the molecular interactions between molecules of the polymer, thereby reducing the brittleness and increasing flexibility of the films (Shaw et al. 2002). More specifically, sorbitol acts to increase the intermolecular spacing between molecules by reducing the hydrogen bonding between internal structures (McHugh and Krochta 1994).

Hexamethylenetetramine

Hexamethylenetetramine (HMTA) is a water-soluble organic compound made via a condensation reaction of organic compounds (Blažzević et al. 1979). It is added to films such as polyvinyl alcohol (PVOH) to act as a cross-linking agent (Ooi et al. 2011; Zhong et al. 2011). Hexamethylenetetramine has also been used in other applications such as medicine.

Cellulose

Cellulose is one of the world’s most abundant natural polymers. It consists of linear chain consisting of anhydro-glucose monomers attached through 1-4 β-linkages (Nazir et al. 2013). In some cases, cellulose can be extracted from fruits such as durian (Penjumras et al. 2016) and oil palm empty fruit bunches (Nazir et al. 2013). Cellulose can be used in many ways in composites applications, such as being used as a reinforcing agent in powder form, nanofibers, and nanocrystals (Penjumras et al. 2016; Gouw et al. 2017).

Petroleum-based Biodegradable Polymers

Some polymers that can be created either entirely or partially from petroleum-based feedstocks can also be biodegradable. Examples of these polymers are poly(butylene adipate-co-terephthalate) (PBAT), PVOH, and poly(butylene succinate) (PBS).

Poly(butylene adipate-co-terephthalate)

Poly(butylene adipate-co-terephthalate) is synthesized from 1,4-butanediol, adipic acid, and terephthalic acid via polycondensation reactions. It has good mechanical properties, which are comparable to the properties of LDPE, though its barrier properties are slightly different from those of LDPE (Costa et al. 2015). Poly(butylene adipate-co-terephthalate) can be processed with normal polymer processing techniques, including mixing, extrusion, and injection molding. The downside to utilizing PBAT is its relatively low degradation temperature (140 °C to 230 °C), which can cause it to degrade during processing (Costa et al. 2015).

Polyvinyl alcohol

PVOH is synthesized from poly(vinyl acetate) by full or partial hydroxylation. The amount of hydroxylation determines the characteristics of the material, such as its chemical properties, physical characteristics, and mechanical properties (Baker et al. 2012). PVOH is often used in medical applications as contact lenses, artificial cartilage, and meniscuses due to its biocompatibility, chemical resistance, high water solubility, and adsorption characteristics (Baker et al. 2012).

Poly(butylene succinate)

Poly(butylene succinate) is a biodegradable polymer that can be synthesized entirely from petroleum or be partially biobased, utilizing biobased succinic acid. It is created from 1,4-butanediol and succinic acid. Its properties are similar to PP and polyethylene (PE) (Fujimaki 1998). Poly(butylene succinate) can be processed easily by injection molding, sheet extrusion, thermoforming, blow molding, and compression molding.

Polycaprolactone

Polycaprolactone (PCL) is a semi-crystalline biodegradable polymer with a low melting temperature of 60 °C and a glass transition temperature of -60 °C. It is synthesized from ε-caprolactone via a ring-opening polymerization reaction. The degradation of the polymer is based on hydrolysis of the ester linkages. It has gained increasing interest in biomedical applications such as implants and drug delivery systems (McKeen 2012), as well as in sustainable packaging (Ahmad et al. 2018).

BIOFILLERS AND FIBERS

Biobased fillers and fibers have been widely used since the 1940s (Mohanty et al. 2000). Biofillers and fibers are advantageous because of their low cost, light weight, and abundance. Incorporating them into a composite increases the amount of biocontent in the composite, which in turn creates a more environmentally friendly product. The use of biofillers also reduces carbon footprints and improves energy security (Mohanty et al. 2018). Biofillers and fibers have other advantages such as reduced tool wear, good thermal properties, reduced worker respiratory irritation, and biodegradability (Mohanty et al. 2000). The downside associated with fillers or fibers is their hydrophilicity, which decreases compatibility with typical polymer matrices that are relatively hydrophobic. The other disadvantage is the low processing temperature to prevent degradation. On average, the thermal degradation temperature for fillers is approximately 180 °C, which limits their use in engineering thermoplastics. Some of these challenges are overcome by modifications, as discussed later.

Fillers

Biofillers reinforce the strength and stiffness of a material. Their properties depend on their source, including what part of the plant they are taken from, the quality of the plant, and its age, among other factors. In this paper, all the fillers discussed are from fruits.

Fillers are typically composed of cellulose, hemicellulose, lignin, and protein because they are not limited to only the fibrous component of plants. Variations in growing season and preparation of the filler can influence its properties, along with the components included in the filler.

Natural fillers considered in this review include apple, banana, acai berry, blueberry, cranberry, coconut, grape, durian, rambutan, olive, mango, pineapple, and date (Table 1). In many cases, the waste is a combination of the skin, flesh components, and seed components. However, some stem materials and leaves would be present and could be processed from these biomass sources.

Table 1. Fruits Used as Filler or Fiber in Composites and Films

Quantity of Fillers

The global production of the various fruits is shown in Table 2. The average waste generated from whole fruits is approximately 30% (Vendruscolo et al. 2008). Wastes generated from apples, grapes, and olives are commonly referred to as “pomace” or “marc,” as they are pressed to extract the liquid components for applications such as making juice or wine.

Table 2. Global Fruit Production in Weight of Whole Fruit

The pomace materials are usually abundantly available but serve very little purpose. In fact, Gouw et al. (2017) estimated that only 20% of apple pomace generated is used for a value-added purpose, whereas the rest remains animal feedstock or is left for compost. Based on the global production of fruit, there are substantial amounts of biomass waiting to be repurposed.

The total waste was calculated by multiplying the global production by the estimated weight percentage of waste, i.e., approximately 30% for fruit. Based on the estimated total wastes generated from fruit processing, there are apparently millions of tonnes of materials in need of a value-added purpose. Currently, some of the waste is used in biorefining and energy production, chemical extraction to generate useful products, or animal feedstocks. However, fruit waste that ends up in landfills or as compost does not possess any sort of added benefit but, rather, generates environmental burden. Therefore, if the biomass is instead used to produce value-added products, then the carbon is sequestered, the impact on the environment is reduced, and the overall sustainability of the materials is improved.

Physicochemical composition of fillers

As mentioned earlier, cellulose, hemicellulose, and lignin are among the major constituents of biobased fillers. Each fruit possesses a unique combination of each of these fractions (Table 3), affected by the agricultural conditions (Muensri et al. 2011). Cellulose is often located within the backbone structure of the fiber. In contrast, lignin is a polyphenolic compound of amorphous structure, located on the outside of the fiber (Muensri et al. 2011). The lignocellulosic materials in the natural fiber are responsible for cross-linking to the polymer matrix. More cross-linking between the filler and the matrix results in better interfacial adhesion, which enhances the filler’s ability to distribute stresses throughout the matrix materials. Better interfacial adhesion often results in improved impact strength of composites (Picard et al. 2019). The lignocellulose content can also affect other properties of the composites.

Table 3. Cellulose, Hemicellulose, and Lignin Contents of Various Fruit Biomasses

Thermal stability of composites is strongly dependent on the presence of natural filler. Thermogravimetric analysis reveals that hemicellulose is the first lignocellulosic material to degrade between 150 °C and 350 °C (Yang et al. 2005). Cellulose and lignin are the next to degrade, at 275 °C to 350 °C and 250 °C to 500 °C, respectively (Kim et al. 2005). It can be inferred that samples with greater hemicellulose contents, relative to cellulose and lignin, would be less thermally stable and therefore be recommended for combining with plastics that have lower melting points.

Fibers

In composite applications, fibers enhance the strength and stiffness of a material. The properties of the fibers depend on their source, i.e., whether they are from the stem of the plant or the leaves of the plant, the quality of the plant, and the age of the plant. Natural fibers can be placed into different groups: leaf, bast, seed, and fruit. This review will focus on fruit leaf and fruit fibers, as they are both by-products of food processing. Natural fibers exhibit variation in length and diameter. Other factors determining the properties are size, maturity, and the processing method used for extraction of the fibers (Mohanty et al. 2000).

Modification of Fibers or Filler

Surface modification of fibers is needed for improving their performance. In composite applications, strong fiber-matrix interaction is crucial for good mechanical properties (Mohanty et al. 2018). However, natural fibers are hydrophilic, which presents a problem when they are added to a polymer matrix that is hydrophobic. Fibers also tend to have a waxy coating on their surface, which causes weak matrix bonding and poor surface wetting (Mohanty et al. 2001). Therefore, surface modification to reduce the hydrophilicity of the fibers can improve the matrix adhesion. Modifications such as washing, peroxide treatment, alkali treatment, bleaching, and the use of silane coupling agents have yielded fiber enhancement (Mohanty et al. 2001). Not only does surface treatment improve the interfacial adhesion, but it can also aid in filler dispersion (La Mantia and Morreale 2011).

Each of the different modification methods has advantages and disadvantages. For example, silane treatments usually result in strong enhancement of mechanical properties. For thermal stability, alkali treatments and acetylation are usually more favorable (La Mantia and Morreale 2011). However, chemical modification is costly and complex, thereby limiting its industrial relevance. Due to these limitations, the most common approach to enhancing natural filler/fiber composites is to add small amounts of compatibilizer to enhance the material properties. The compatibilizer should have both hydrophilic and hydrophobic moieties, which interact with polar groups on the filler’s surfaces and polymer molecular chains, respectively. Although the addition of compatibilizer is common, it is not the focus of this review.

Washing

Washing of fibers is a routine step when modifying fibers. Washing fibers in water alone or with water and a mild detergent removes dust or any other impurities that could be present. Fibers are usually also washed after chemical modification to remove any residual chemicals. Washing has also been used to remove excess free sugars in some biomass sources to improve the thermal stability at polymer processing temperatures.

Bleaching

Bleaching of fibers is done to remove colour and/or to modify the surface properties. In lignocellulosic fibers, the colour of the fibers can be associated with lignin (Razak et al. 2014). Bleaching also enhances mechanical properties, as it can increase the surface roughness of the fibers, resulting in stronger fiber-matrix adhesion (Razak et al. 2014).

Peroxide treatment

Peroxides are molecules with an O-O group. Peroxides can decompose easily, forming free radicals in the form of RO·, which then react with the hydrogen groups on the surface of the fibers. For example, the hydrogen on the cellulose (Cell-H) interacts with RO·, which in turn can react with the polymer as shown in Eq. 1 (Joseph et al. 1996):

RO· + “Cell – H” → ROH + Cellulose·

Polymer· + Cellulose· → Polymer – Cellulose (1)

Peroxide treatment can remove surface impurities, hemicellulose, and lignin and can also increase the surface roughness of the fibers, increasing the fiber-matrix interaction (Razak et al. 2014). For peroxide treatments, fillers/fibers are added to a peroxide solution and left to soak. The fillers/fibers are washed to remove any residual peroxide and then dried.

Alkali treatment

Alkali treatment, also called “mercerization” if the concentration is high enough, is a low-cost and effective method for surface modification of fibers. The reaction between sodium hydroxide (NaOH) and cellulose fiber (Cell-OH) is shown in Eq. 2 (Mohanty et al. 2001):

“Cell – OH” + NaOH → “Cell – O^–Na⁺” + H₂O + surface impurities (2)

This treatment is effective in removing lignin, wax, and oil that cover the external surface of the fiber, depolymerizing the cellulose structure, and exposing crystallites of short length (Mohanty et al. 2001). It also increases the surface roughness by disrupting the hydrogen bonding in the network structure (Li et al. 2007). During alkali treatment, fibers are immersed in NaOH solution. The fibers are then removed and dried. The NaOH concentration and soaking time need to be optimized because high NaOH concentration or extended soak duration can result in undesired fiber characteristics (Li et al. 2007). Typically, low concentrations of NaOH (approximately 5% or less) are incorporated with water in a bath. The filler/fibers are then soaked for a short duration, from 30 min to a few hours. The filler/fibers are then washed to remove the NaOH and dried to remove moisture.

Acetylation treatment

Acetylation is a type of esterification method that introduces an acetyl group into a compound. The acetic anhydride addition to the lignocellulosic components of the biofiller/fiber causes an esterification reaction with the hydroxyl groups in the cell walls of the biofiller/biofiber (Rowell 2004). Acetic acid is formed as a by-product of the reaction. This reaction is shown in Eq. 3 (Rowell 2004):

“Cell wall – OH” + “CH₃C(= O) – O – C(= O) – CH₃” →

“Cell wall – O – C(= O) – CH₃” + “CH₃C(= O) – OH” (3)

For acetylation, biofillers/fibers are soaked in water and then filtered and placed in an acetylation solution. The temperature, duration, and exact chemicals used can vary, but the fillers/fibers are typically washed and dried after this process (Bledzki et al. 2008)

Silane treatment

Silane is an inorganic compound with the chemical formula SiH₄. Silane may reduce the amount of cellulose hydroxyl groups on the surface of the filler and reduces the fiber-matrix interaction (Li et al. 2007). Silane reacts with water and forms silanol and alcohol. The silanol then reacts with the OH groups on the fiber, resulting in stable covalent bonds on the surface of the fiber (Agrawal et al. 2000). The proposed reaction is shown in Eqs. 4 and 5 (Agrawal et al. 2000):

CH₂CHSi(OC₂ H₅)₃ → CH₂CHSi(OH)₃ + 3C₂H₅OH (4)

CH₂CHSi(OH)₃ + “Fiber – OH” + H₂O → “CH₂CHSi(OH)₂O – Fiber” (5)

The swelling of the fiber is prevented because the silane creates a crosslinked network of covalent bonds between the fiber and matrix (Li et al. 2007). There are many different silanes used, along with different concentrations, such as 1 M NaOH for 1 h at room temperature (Jandas et al. 2011). For example, long-chain alklyl methoxysilanes can be used to prepare hydrophobic plant-based fibers (Sgriccia et al. 2008). For silane treatments, the fillers/fibers are soaked in a mixture containing a small amount of silane, with a mixture of water and ethanol or a solvent mixture such as toluene/ethanol/acetone (Shih et al. 2014; Hemsri et al. 2012). After soaking for a specific duration, the fibers are washed and dried. The specific conditions depend on the silane used and the desired properties.

Plasma treatment

Plasma treatment is a rather recent development for changing the surface properties of natural fillers/fibers without significantly affecting their bulk properties (Kalia et al. 2009). A plasma treatment is based on allowing ionized gas with an equal number of positively and negatively charged molecules to react with the surface of the fillers/fibers (Kalia et al. 2009). The treatment conditions used heavily influence the final properties of the treated material, so it is difficult to generalize the final properties (La Mantia and Morreale 2011). Podgorski and Roux (1999) and Podgorski et al. (2000) determined that the type of gas, the treatment duration, the power, and the distance between the samples and the plasma source all influenced the properties of the plasma-treated biofillers/fibers.

PROCESSING METHODS

Processing methods for the production of composites with plant-based fillers include injection molding, compression molding, and compounding (Rout et al. 2001; Jandas et al. 2012; Dong et al. 2014; Koutsomitopoulou et al. 2014; Hassaini et al. 2017). The method chosen usually depends on how the incorporation of the biofiller/fibers is performed. Woven fibers are usually processed with compression molding, while short fibers are usually injection molded. The end application is also considered when determining which processing method to use. The processing method chosen also reflects the thermal stability of the added biofiller/fiber, as many naturally sourced materials have low thermal stability, limiting processing temperatures to less than approximately 200 °C.

Injection Molding

Injection molding is a popular processing method for plastics due to the wide variety of materials that can be used and its ease of use. Injection molding is often used in composite applications because it has been shown to improve the fiber dispersion and increase the tensile and flexural properties (Mohanty et al. 2004). Conversely, injection molding can reduce fiber lengths due to the high shear the fibers are exposed to during the extrusion and injection conditions, which in turn changes the properties of the composites.

Compression Molding

Compression molding is a widely used composite fabrication technique due to its low cost and simplicity. One of the major benefits of compression molding is the amount of control over fiber orientation. The fibers can be randomly orientated, creating a composite with isotropic properties, or they can be selectively orientated, as desired. Certain natural fibers can be difficult to disperse during compression molding.

Every different composite formulation will have a preferred processing method. This can be determined based on previous work with similar polymer matrices and biofiller/fiber type, or by trial and error. Processing techniques include creating layers of a fine polymer powder between a biofiller/fiber, sandwiching long fibers between layers, sandwiching short fibers or fillers between layers, dry mixing of components, and multiple layers of both powdered polymer and fibers until the desired weight percentage of material is used (Akil et al. 2011).

Casting

Casting is a common method used to make films. Casting requires the preparation of a polymer solution, which is poured into a mold and dried under ambient conditions or oven-dried at an elevated temperature. Different materials are used in combination with water or other solvents to form the solution for film production. These materials include starch and PVOH. Starch is a low-cost, versatile, and readily available biodegradable material with excellent film-forming properties (Luchese et al. 2018). Polyvinyl alcohol, however, is a water-soluble polymer that is formed via polymerization and hydrolysis of vinyl alcohol. Polyvinyl alcohol is a biodegradable polymer with high film strength. Plasticizing agents, such as sorbitol and glycerol, are being used to impart flexibility to the final film structures (Luchese et al. 2018).

BIOCOMPOSITES AND FILMS

Biocomposites derived from both natural fillers/fibers and biodegradable plastics can be classified as biodegradable materials. Despite much research on natural fiber composites, studies related to incorporating food waste into biocomposites are limited. Most of the research related to biocomposites from food waste focuses on biomasses such as coconut, olive, pineapple, and banana. Composites derived from these biomasses and other fruits are discussed in detail, focusing on their mechanical and thermal properties. The properties of the biocomposites depend heavily on the type of polymer matrix used, the treatment of the fibers, the amount of fibers added, the potential addition of compatibilizer, and the process techniques, among other factors. In general, the addition of compatibilizer and surface treatment of the biofiller/fiber typically improved the mechanical properties of the biocomposites.

Fruit Wastes in Composites

Various fruit wastes, such as blueberry, cranberry, apple, acai berry, rambutan, kiwi, mango, palm fruit, date fruit, and jackfruit, have been used to generate biocomposites and films.

Park et al. (2009) created blueberry, cranberry, and grape pomace biocomposite boards (Fig. 2) with soy flour modified with NaOH. They found that the blueberry pomace had the greatest breaking strength and modulus of elasticity values, as compared to the other pomace biocomposites. The addition of glycerol, acting as a plasticizer, increased the flexibility and decreased the stiffness of the test specimens (Park et al. 2009). The degradation temperature shifted slightly lower with the addition of the blueberry pomace, which was attributed to the lower thermal stability of the biofillers.

Fig. 2. (a) Blueberry, cranberry, and grape pomaces and their corresponding composites, (b) scanning electron microscope (SEM) image of fracture surface of blueberry pomace board. Reprinted with permission from John Wiley & Sons: Journal of Applied Polymer Science, (Park et al. 2009), License Number: 4561941149394.

Wataya et al. (2015) developed acai fiber-filled PBAT/PLA biocomposites. They found that 15 wt% of acai fiber decreased the tensile strength, Young’s modulus, and impact strength. However, the elongation at break increased by approximately 17%. The melting temperature remained practically unchanged with the addition of fiber (from 178.0 °C to 178.5 °C) (Wataya et al. 2015). Apple pomace (AP) can be generated from bruised apples, jam production, and juice/cider production. This material is abundant around the world (Shalini and Gupta 2010). Gaikwad et al. (2016) created AP/PVOH films with antimicrobial properties. The AP/ PVOH film with 30 wt% AP had 40% scavenging activity on a free radical scavenging assay when using 2,2-diphenyl-1-picrylhydrazyl (DPPH), whereas the control with just a PVOH film had no scavenging activity. This result indicated that the AP films had a high antioxidant capacity. The authors attributed this result to the phenolic compounds within the AP (Gaikwad et al. 2016).

AP fiber was studied by Gowman et al. (2019) via scanning electron microscopy (SEM), showing porous and sheet-like morphologies (Fig. 3a). The different morphologies were attributed to the components of the pomace, including skin, seed, and flesh materials (Gowman et al. 2019). Picard et al. (2019) combined AP with biobased PBS (BioPBS) to generate biocomposites, with maleic anhydride-grafted BioPBS as a compatibilizer to improve the mechanical properties. The researchers found that the addition of AP improved the impact strength by more than 120%, even without the addition of compatibilizer. An SEM image of these composites is shown in Fig. 3b. The flexural strength and modulus of the composites with compatibilizer were greater than those of the neat BioPBS. Furthermore, SEM analysis determined that the addition of the compatibilizer improved the interfacial adhesion of the samples (Fig. 3c). The AP-based sustainable biocomposites may find single-use applications for food.

Fig. 3. (a) AP fiber (Gowman et al. 2019), (b) AP composites (Picard et al. 2019), (c) AP-compatibilized composites. Reprinted with permission from Springer Nature: Waste and Biomass Valorization, (Picard et al. 2019), License Number: 4560191136299.

Luchese et al. (2018) created blueberry pomace and cassava starch films. They found that the addition of blueberry pomace to their films caused high absorption of wavelengths less than 300 nm, meaning that the pomace was able to protect the films from UV light. The improved UV resistance was attributed to the presence of aromatic compounds in the blueberry pomace, which could help extend the shelf life of foods if used for food packaging applications.

Mittal et al. (2015) created date seed powder (DSP) and PLA/PBAT biocomposites. The authors found that 40 wt% of DSP increased the tensile modulus of their PBAT biocomposites by more than 300%, while with a PLA matrix, an increase was seen until reaching 20 wt% (Mittal et al. 2015). The melting point for the PBAT composites decreased with increasing DSP, but it remained almost the same for the PLA composites (Mittal et al. 2014). Voids were present and were explained by moisture evaporation during processing. The authors also noticed oil migration from the DSP but did not notice a change in the mechanical properties of the composites (Mittal et al. 2015). The biodegradation was improved with the addition of the DSP. Increased biocontent is known to improve degradation of composites, and in this case it resulted in bigger cracks and surface degradation in the samples (Mittal et al. 2014).

Rambutan (Nephelium lappaceum) is a tropical fruit that consists of an outer layer, flesh, and seed components. Cast films were prepared with PVOH, glycerol, HMTA, polysorbate 80 as a matrix material, and rambutan skin flour at approximately 8 wt% to 32 wt% filler content. Increasing rambutan flour content resulted in decreased tensile strength. Other properties such as water absorption were increased due to the hydrophilic nature of the filler. This can be less favourable for the use of the composite in food packaging or other applications. However, rambutan did show enhanced adhesion and stronger interaction with the matrix material than did banana skin flour in the same study (Zhong et al. 2011).

Durian (Durio zibethinus) is a commonly consumed fruit in Southeast Asian countries. It consists of 50% to 65% flesh components, and the remaining skin and seed materials are waste. Manshor et al. (2014) reported that NaOH-treated durian skin fibers possessed higher impact strength than untreated samples. However, there was little improvement to the overall impact strength of PLA. The authors concluded that the overall performance was comparable to PLA and suggested the use of this filler for cost-reduction (Manshor et al. 2014). In another study, durian husk fiber was combined with PLA at 15 parts per hundred rubber (phr), 30 phr, 45 phr, and 60 phr (Lee et al. 2018). The mixtures were compression molded to generate tensile samples. Although tensile strength was lower in the composites as compared to the neat polymer, an increasing tensile strength was observed with increasing fiber content. Furthermore, the tensile modulus increased beyond that of the neat polymer as the fiber content increased. The authors highlighted that the samples were quite brittle (Lee et al. 2018). Thus, further work needs to be done to enhance the interactions of the fiber and matrix and further improve the mechanical properties of the composite.

Table 4. Fruit Pomace and Fruit Waste Composites