Encyclopedia
Please note this is a comparison between Version 1 by Young-Kee Kim and Version 2 by Nora Tang.
Since the invention of plastics and the development of mass production, plastic demand has increased exponentially annually. Despite their easy processability and economically viable merits, the management of plastic waste has always been a contentious issue due to their non-biodegradable properties, resulting in significant damage to the environment. Although the share of bioplastics in the plastic market remains low, eco-friendly and abundant amounts of bioplastics are considered to be sufficient alternatives to replace preexisting plastics. Chlorella and Spirulina are the primary sources for bioplastics from microalgae due to their easy processability. Although the replacement of artificially synthesized existing plastics with 100% microalgal bioplastics could be ideal when it comes to environmentally friendly plastics, the physical and mechanical properties, such as tensile strength, glass transition temperature, and elongation at break, of 100% bioplastics are inferior to those of commercially available plastics. Another option to tackle these issues is to blend microalgae with polymers, namely microalgal biomass-polymer composites. These blended composites are hybrid-type plastics that have both properties of each component. The general properties of these blends depend on the ratio between the amount of microalgae biomass and those of the polymer. In general, a higher polymer content in biomass-polymer composites results in better performance, especially in terms of tensile strength and elongation at break. The required level of biomass contents in authorized bioplastics depends on individual national regulations. Korean government suggest that the percent of biomass in biomass-polymer composites should be over 30 wt% to be considered as an eco-friendly bioplastic. According to our investigation, the contents of microalgae in most composites remain less than 30 wt%, implying that further studies to increase the proportion of microalgae in the composites should be conducted to boost the commercialization of microalgae-based biocomposites.
- microalgae
- biomass
- biopolymer
- biocomposites
- bioplastics
1. Composites with Poly(Vvinyl Aalcohol)
Lipids, one of the most valuable components in microalgae, can be extracted using simple methods that induce direct physical forces, such as microwaves and sonication [1][2][3][56,57,58]. Despite their simplicity, small amounts of lipids from microalgae hinder their utilization in industry. Lipids extracted from the microalgae of Nannochloropsis salina lead to the mass production of oil from algal biomass [4][5][59,60]. In this method, lipid-extracted microalgae are often used as the filler to make composites with polymers. Tran et al. attempted to blend lipid-extracted microalgae with poly(vinyl alcohol) (PVA). PVA is one of rare water-soluble synthesized polymers. In contrast to the direct blending method, which involves blending microalgae with a heated polymer by increasing the temperature to the glass transition temperature of the polymer, PVA can be dissolved in water to mix microalgae. PVA is easily dissolved in protic solvent at 90°C. The characterization of functional groups for microalgae is essential to check whether microalgae is compatible with petroleum polymer. The measurement of infrared [IR] spectrum in Figure 12 reveals that lipid-extracted microalgae have strong peaks at 3285 cm−1, 2919 cm−1, and 2851 cm−1, which are assigned to O–H, CH2, and CH stretching vibration, respectively [6][54]. The use of PVA, which has exactly same functional groups, leads to relatively homogeneous mixing with lipid-extracted microalgae by chemical interaction between polymeric metrices. The resulting biocomposites displayed enhanced thermal stability and mechanical properties when the portion of lipid-extracted microalgal biomass to PVA was 20%. However, the relatively high cost of extracting specific components from N. salina may not be adequate for the ultimate commercialization of these biocomposites.
The Dianursanti group also reported on blends of microalgae with PVA. They focused on using whole microalgae (Spirulina platensis) to mix with PVA. S. platensis, which contains a large portion of protein (approximately 60 wt%), has a positive effect on elongation at break when it forms biocomposites with PVA. They used 56% of microalgae to make composite and added glycerol as a plasticizer, which led to further improvements in the tensile strength [7][61]. The use of whole microalgae with PVA resulted in enhanced plastic properties of composite and cost performance, minimizing the processing costs. In addition, the limitations of conspicuous enhancement of properties prevents them from competing with preexisting plastics in markets. In addition, they treated PVA with maleic anhydrate as compatibilizer to make maleic anhydrate-grafted PVA (PVA-g-MAH), and PVA-g-MAH was blended with Chlorella vulgaris for fabricating biocomposite to overcome the above mentioned drawbacks [8][62]. PVA-g-MAH was synthesized using a simple and economical method by mixing maleic anhydrates, dimethyl sulfoxide, and potassium persulfate with PVA. The biocomposites composed of C. vulgaris with PVA-g-MAH increased the elongation at break and tensile strength. Furthermore, elasticity was also improved, in contrast with that achieved using other methods. Another simple method to form biocomposites is the sonication of Chlorella, followed by mixing with PVA [9][63]. However, despite slight improvements in tensile strength and elongation at break, the degree of improvements compared to other methods was marginal.
2. Composites with Polyethylene or Polypropylene
Polyethylene (PE) and polypropylene (PP) are poly(olefins) used in a variety of products owing to their rigidity, easy processability, and low cost. The application of these polymers to microalgae to form biocomposites has been attempted at the very beginning of research on biocomposites. The use of whole microalgae rather than extracted components from microalgal biomass was dominant to blend with PE and PP [10][11][29,64]. Despite their easy accessibility and viable processes, the intrinsic structural differences of these polymers lead to incompatibilities, in particular for Chlorella. Further studies on enhancing compatibility will be needed.
So far, the purpose of using microalgae blended with polymers has mainly focused on mechanical properties improvement. However, a recent report regarding the use of microalgae in PP has emphasized the stabilization effects [12][65]. They reported that the stabilization effects by adding Chlorella vulgaris and Spirulina platensis in PP. Microalgal biomass acted as agents to protect the degradation of PP, primarily caused by polyphenols in microalgae. Tafreshi et al. also reported that polyethylene glycol (PEG), which is chemically different from PE, combined with C. vulgaris significantly improved the stress against gamma irradiation [13][66]. These recent studies have demonstrated the potential of biocomposites with simple polymers (such as PE and PP), with a focus on stability issue.
3. Composites with Poly(Vvinyl Cchloride)
Poly(vinyl chloride) (PVC), which has a high density, hardness, and durability, has also been used to fabricate biocomposites with microalgae. A simple method to blend PVC with microalgae involves pressurized heating by increasing the temperature to 190°C [14][67]. However, the application of relatively high temperatures to PVC blended with Chlorella may have negative effects, leading to weight loss of Chlorella due to the volatilization and degradation of chlorophyll in microalgal biomass. Despite these drawbacks, by handling with care, the addition of Chlorella to PVC enhances the tensile strength. In their study, the weight ratio of Chlorella, as filler, to PVC was less than 20% to meet the requirements for rigid PVC products.
4. Composites with Polyurethane
Polyurethane (PU) is a polymer synthesized by reacting isocyanate and polyol groups. Despite their superior flexibility, their relatively low hardness and strength serve as a barrier to their use with microalgae. The applications of PU in biocomposites have focused on biomass with high lignin content because of the limited properties of PU [15][68]. The actual use of PU blended with microalgae has recently been studied [16][69]. A mixture of PU and Chlorella treated with sonication improved the mechanical properties of the biocomposites. The increase in the ratio of Chlorella to PU enhanced the tensile strength and elongation at break, similar to those of other polymer blends. Large amounts of Chlorella (up to 70 wt%) were used with the help of PEG as a model polyol, which is the highest content compared to other reports regarding biocomposites. The optimization of biocomposites using PU and microalgae as biofillers will be needed to further improve their applications.
The further characteristics of biocomposites mentioned-above are summarized with respect to pros and cos in Table 12.
Table 12. Summary of various microalgae-based biocomposites.
| Microalgae | Blended Polymer | Pros | Cons | Ref. |
|---|---|---|---|---|
| Lipid extracted Nannochloropsis salina | PVA |
|
|
[6][54] |
| Chlorella vulgaris | PVA-g-MAH | - Enhanced tensile strength and elongation properties | - Complex compatibilizer required | [8][62] |
| Chlorella vulgaris and Spirulina platensis | PP, PE |
|
- Incompatibility especially with Chlorella | [10][29] |
| Chlorella | PVC | - High tensile strength due to rigidity of PVC | - High sensitivity to water content in microalgae | [14][67] |
| Chlorella | PU | - Superior mechanical properties due to interactions with isocyanate group | - Complex process requiring a coupling reagent | [16][69] |
5. Biodegradable Bioplastics
In terms of biodegradability of polymers, poly(butylene succinate) (PBS) is considered as eco-friendly polymer. Although its thermoplastic properties are similar to those of PE, allowing for blending with microalgae using high temperatures, studies on composite with PBS have rarely been conducted, presumably due to technical issues [17][53]. They attempted to form maleic anhydride-grafted PBS (PBS-g-MAH), similar to PVA-g-MAH conducted by Dianursanti and Khalis [8][62]. As a compatibilizer, PBS-g-MAH was found to improve the mechanical properties of eco-friendly composites when blended with Spirulina.
Polyhydroxyalkanoate (PHA) is a polyester that is widely known as a biodegradable plastic. Poly-3-hydroxybutyrate (PHB) is a type of PHA with properties similar to those of PP and PE with self-degradable effects. Recently, it has been reported to the extraction of PHB from microalgal biomass [18][19][20][70,71,72]. In addition to Chlorella and Spirulina, other types of microalgae, such as Chlorogloea fritschii, have been used to extract PHB [18][70]. In general, valuable components extracted from microalgae are not used unilaterally but are blended with other materials (mainly polymer). As PHB can solely be utilized as a plastic because of its similarity to commonly used plastics, the extraction of PHB from microalgae has been an important focus of the production of biodegradable polymers.
