The heavy reliance of the food industry on flexible packaging for its beneficial qualities is unlikely to diminish in the foreseeable future. In fact, a conservative model predicts that the production of plastic will increase by 10.8 million tons each year in the next decade (Brandon and Criddle 2019). This by far outstrips the estimated 0.8 million tons of plastic that are recycled each year in a time of increasing public dissent and decreasing landfill capacity. To combat this issue, many biopolymer-based plastics have been developed as alternatives to conventional petroleum-derived plastics in the packaging industry. However, performance, processing, and costs are the main hurdles that need to be addressed if these biopolymers are to replace conventional polymers on a large scale. For instance, the costs of biopolymers range from 3–10 times the price of conventional polyolefin plastics (Rodriquez-Perez et al. 2018). Biopolymers also struggle to meet the desired mechanical strength, thermal stability, molecular weight (MW) distribution, melt strength, and modulus needed for processing, as well as the barrier and water resistance required for consumer product protection (Lucas-Freile et al. 2018; Mensitieri et al. 2011; Petersen et al. 1999).
Biopolymer-based plastics may exhibit inadequate shelf-life performance, poor mechanical properties, and low thermal stability (Brandon and Criddle 2019). These limitations make food protection an issue, and they can be challenging to run on typical polymer processing, converting, and packaging equipment. The very properties that make a film compostable (such as structures that are subject to hydrolytic attack) often undermine its ability to be run efficiently on machinery such as blown, cast, extrusion, print, and package lines. Additionally, biopolymers generally have poor oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) when compared to oil-derived plastics. Poor performance in these metrics negatively affects the shelf-life of some products. However, proven processes, such as the addition of nucleating agents to increase crystallinity, can rectify some of these weaknesses. Therefore, more research must be conducted to generate sustainable nucleating agents that can be utilized as additives in sustainable packaging structures. Sustainable additives can thereby enhance the desired properties of the sustainable matrix while not taking away from the degradability of the composite.
A logical and cost-efficient sustainable additive would be the most abundant naturally occurring polymer on the planet, cellulose (Habibi et al. 2010). It is located in the cell walls of many biomasses (plants, bacteria, tunicates, etc.), making it a sustainable and renewable replacement of conventional polymers produced mainly from limited resources (Chakrabarty and Teramoto 2018; Yang et al. 2007). This biopolymer is available in various terrestrial and marine environments. In its native forms, cellulose exists in a matrix of largely composed of lignin and hemicellulose. To isolate cellulose, alkaline agents (e.g., caustic soda) and weak acids (e.g., acetic acid) have been used to solubilize and extract the non-cellulosic parts (Doh et al. 2020). However, these reactants will not reduce cellulose down to the mechanically superior cellulose nanocrystal (CNC), which has many advantages in the packaging industry due to high tensile strength (7.5–7.7 GPa) and elastic modulus (110–220 GPa with an average of around 170 GPa) while maintaining a low density (1.61 g/cm) (Habibi et al. 2010; Stark 2016). Furthermore, the high functionality of CNCs enables them to act as nucleating agents and increase the crystallinity of the surrounding polymer (Habibi et al. 2010). Thus, to fully leverage the mechanical strength of the cellulose molecule, amorphous factions of cellulose and non-cellulosic material must be discarded to isolate the highly crystalline and mechanically useful CNC.
The primary methods used to produce CNCs are enzymatic and acid hydrolysis. Following depolymerization and bleaching steps to remove non-cellulosic materials, this research will utilize acid hydrolysis to produce CNCs. The general process is illustrated in Fig. 1. During acid hydrolysis, the hydronium ion, produced by the hydrolyzing sulfuric acid, transversely breaks the glycosidic bonds of the accessible amorphous regions (Mozdyniewicz et al. 2016). Theoretically, the glycosidic break leaves the tightly packed crystalline micelle domain intact and substitutes sulfate esters throughout the CNCs (Khalil et al. 2015). These negatively charged sulfate groups are critical in minimizing aggregation (via repulsion forces) and forming stable solutions (Doh et al. 2020; Habibi et al. 2010; Wang et al. 2018). Sulfuric acid hydrolyzed CNCs self-assemble into a chiral nematic ordered phase in film and solvent suspensions (Habibi et al. 2010). The chiral nematic order of CNCs can be leveraged to design specific optical characteristics. Implementations of CNCs into a matrix can increase optical clarity up to a critical concentration (Chakrabarty and Teramoto 2018). However, being a hydrophilic non-thermoplastic polymer that tends to agglomerate, it is intrinsically difficult to extrude CNCs when compounded with the typical polymers in the sustainable packaging industry. Issues have included the molten curtain frequently breaking, the CNCs thermally degrading, and the CNCs significantly agglomerating during processing (Oksman et al. 2016). So, although it has gained academic notoriety (where small-scale solvent casting is the preferred method), it has yet to be widely adopted in large-scale industrial processes (Oksman et al. 2016). Nevertheless, sulfated nanocrystals have shown to decrease agglomeration, and recent advances in continuous melt processing such as co-rotating twin-screw extrusion, solid-state pulverization, grafting, and chemical pretreatment are making up for compounding deficiencies (Oksman et al. 2016; Stark 2016). Thus, the two main issues in obtaining useful CNC composites are (1) removing non-cellulosic material from the biomass and amorphous factions from the native cellulose and (2) dispersing the CNCs throughout a matrix without significant agglomeration during processing.
This research will utilize kudzu (Peuraria montana var. lobata) as the biomass for CNC extraction. Kudzu, infamously known in the southeastern United States as “the vine that ate the South,” is an invasive species in the United States that overtakes and strangles surrounding due to its rapid growth (Gulizia and Downs 2019; Keung 2002). In its native environments (China, Japan, and other parts of Asia), the rapidly growing vine is kept at bay by numerous evolutionary counterforces. However, since kudzu has only been in the United States since the late 19th century, the vine is free to spread rapidly without much competition. It is a suffocating, semi-woody vine that is a strong climber that overtakes surrounding vegetation and man-made structures to achieve maximum photosynthesis. The vine exhibits diameters between 1-2.5 cm (Keung 2002). Three to five vines originate from a crown that sits in the topsoil connected underneath by bulbous and tubular-shaped roots that form a sizeable horizontal network one to three meters below the ground (Shurtleff and Aoyagi 1977). These large starchy roots (2–18 cm in diameter) contain an abundant energy reservoir that makes it a drought-resistant and fast-spreading plant in the warm months. The roots also firmly entrench the legume into the surrounding soil (Keung 2002; Shurtleff and Aoyagi 1977). The deeply entrenched roots and drought-resistant nature of kudzu make it a good candidate for preventing soil erosion. Nevertheless, the properties that make kudzu such a hardy plant also come with the trade-off of being extremely hard to exterminate and control (Keung 2002; Shurtleff and Aoyagi 1977). Given its exceptionally rapid propagation, it is now widely considered an agricultural nuisance. It is safe to say its removal and harvesting for alternative purposes would be welcomed by local populations in the United States. A newly found use for this noxious weed would significantly improve its perception, help regulate its proliferation, and provide a renewable resource to whichever industry can find it a value-added purpose (Luo et al. 2002).
This research believes that the packaging industry can use kudzu for CNC extraction. It is important to note that the properties of CNCs depend on the source, time of harvest, extraction methodology, mechanical treatment, chemical treatment, and scale of the cellulosic material (Khalil et al. 2015). Many studies have been conducted to determine the most effective source for nanocellulose extraction as shown in Table 1 (Chakrabarty and Teramoto 2018). Logically, higher cellulose compositions would lead to higher CNC yields. Furthermore, a higher aspect ratio is important when incorporating the CNCs into a polymer matrix to enhance mechanical, thermal, and barrier properties (Clyne and Hull 2019; Doh et al. 2020). Doh et al. (2020) demonstrated that two other plentiful but problematic species, the Sargassum natans (also known as sargassum seaweed) and Laminaria japonica (also known as kombu seaweed), produce CNCs with treatment and enhanced the mechanical and thermal properties of the composite. The sargassum seaweed contains approximately 20% cellulose, and the kombu seaweed contains 17% cellulose (Rabemanolontsoa and Saka 2013; Shi et al. 2011). Hence, even relatively low cellulose comprising sources can produce useful CNCs. Cellulose makes up 33% of the kudzu vine composition (Anele et al. 2020; Luo et al. 2002; Tanner et al. 1979; Wilke and Rosenberg 1977). The rest of the vine is composed of hemicellulose (11%), lignin (14%), solubles (41.4%), and ash (0.3%) (Luo et al. 2002; Tanner et al. 1993). However, the kudzu aerial parts (leaf and stem) contain less cellulose (14–20%), less lignin (4–6%), and more hemicellulose (10–20%) on average when compared to the vine (Anele et al. 2020; Gulizia and Downs 2019). The composition percentages is highly dependent on if the kudzu was harvested in the early or late season (Gulizia and Downs 2019; Uludag et al. 1996). Furthermore, the cellulose of kudzu has been reported to have a relatively high complex viscosity and a high degree of polymerization (DP) above 1720 (Harland 1952; Li 2003). With sufficient acid concentration, the DP will reach a level-off degree of polymerization (LODP) independent of the source's initial DP before hydrolysis (Mozdyniewicz et al. 2016; Hamad and Hu 2010). Moreover, Hamad and Hu also show that crystallinity reaches a maximum of 90% around the LODP, and the nanocrystal reaches a minimum diameter of 8–10 nm, independent of the initial DP (Mozdyniewicz et al. 2016). Thus, kudzu, given its relatively high concentration of cellulose and its large DP, may present a viable biomass for the extraction of CNCs with relatively high aspect ratios.
1.5. Research Purpose
Due to the growing interest in biopolymers, the purpose of this research is to determine the efficacy of extracting nanocellulose from kudzu, characterize said nanocellulose, and compare it with extracted nanocellulose from other sources, such as seaweed biomass (Doh et al. 2020). The potential industrial consumption of a predominantly troublesome and invasive species clearly has its inherent advantages, especially with its target use being the growing sustainable packaging market. Numerous biomasses (cotton, hemp, flax, spruce, bamboo, seaweed, etc.) used acid hydrolysis to extract nanocellulose with varying degrees of concentration, time, temperature, and type of acid (Bondeson et al. 2006; Brito et al. 2012; Chen et al. 2016; Doh et al. 2020; Donaghy et al. 1990; Dong et al. 1998; Hamad and Hu 2010; Lu and Hsieh 2010; Luzi et al. 2014; Mondragon et al. 2014). However, according to the literature review of this field, the process to isolate nanocellulose from kudzu has never been documented, and consequently, its resulting characteristics have yet to be formally studied. Thus, the first objective of this research is to evaluate the effectiveness of previous isolation methods on two parts of kudzu biomass: the aerial and vine regions. The second objective is to characterize the resulting isolated nanocellulose particles and compare to previous sources of nanocellulose extraction to discern if the kudzu nanocellulose can provide any value to the sustainable packaging landscape
Table 1
Compositions of previous used sources for CNC extraction and their resulting aspect ratios ([1] Anele et al. 2020; [2] Doh et al. 2020; [3] Filson et al. 2009; [4] Gulizia and Downs 2019; [5] Khalil et al. 2015; [6] Lou et al. 2002; [7] Luzi et al. 2016; [8] Rabemanolontsoa and Saka 2013; [9] Shi et al. 2011; [10] Sundarraj and Ranganathan 2018; [11] Tanner et al. 1979; [12] Wilke and Rosenberg 1977).
Source | Composition (%) | Aspect Ratio (L/D) | References |
Cellulose | Hemicellulose | Lignin |
Cotton | 95 | 2 | 1 | 10 | 5, 10 |
Flax (retted) | 71 | 21 | 2 | - | 5, 10 |
Flax (unretted) | 63 | 12 | 3 | 15 | 5, 10 |
Hemp | 70 | 22 | 6 | 35 | 5, 7, 10 |
Kombu | 17 | 31 | 0 | 11 | 2, 9 |
Kudzu (aerial) | 14–20 | 10–20 | 4–6 | ? | 1, 4 |
Kudzu (vine) | 33 | 11 | 14 | ? | 1, 6, 11, 12 |
Recycled Pulp | 98 | 1 | 1 | 3–23 | 3 |
Sargassum | 20 | 43 | 8 | 6 | 2, 8 |
Sisal | 73 | 14 | 11 | 60 | 5, 10 |
Sugarcane Bagasse | 40 | 30 | 20 | 64 | 5, 10 |
Wheat straw | 30 | 50 | 15 | 45 | 5. 10 |
Wood | 40–47 | 25–35 | 16–31 | 50 | 5, 10 |