This section has been divided into several subsections: bioplastic materials, bioplastic manufacturing process, bioplastic test properties, frequently used keywords in the field of bioplastic; and most productive, collaborative, and cited country, organization, author,journal, publications in the field. The later sections aredivided into several subsections: most productive, collaborative, and cited countries; most productive, collaborative, and cited organizations;most productive, collaborative, and cited author; most productive and impactful journals; most impactful publications. The application of bioplastic and future perspectives and limitations are described in the last section. Overall, the research on bioplastics has increased substantially. Figure 3 shows the exponential growth of the study concerning bioplastics, from 45 in 1999 to 542 publications in 2020, indicating the subject matter's importance.
3.1 Bioplastics Material
Bio-based materials (viz. polysaccharides, proteins, lipids, biobased polyesters, microbial polymers, and synthesized biopolymers) with additives (viz. plasticizers, stabilizers, antimicrobial agents, antioxidant agents, flame retardants), etc., have been used to manufacture bioplastics. A minimum of one occurrence of keywords, one item per cluster, and a maximum of 10,000 lines were considered to identify the widely used materials for bioplastic manufacturing. Based on that, from the 12,446 keywords, 1,679 were manually selected related to manufacturing materials and bioplastics. Further, out of 1,679 keywords, 1,503 keywords showed a link between them. Thus, the bibliometric map of 1,503 keywords containing various materials used by researchers is provided in Fig. D.1 of Annexure D (Supplementary).
The diverse publications used different notations for the same material, e.g., polylactic acid was written as poly (lactic acid) or poly(lactic) acid or polylactic acid or polylacticacid (PLA). Thus, the total number of occurrences was taken as the summation of all different notations. Likewise, keywords related to various sources of starch, cellulose, materials used as a stabilizer, materials used as a plasticizer, sources of fibers, and proteins are covered under the head to starch, cellulose, stabilizers, plasticizers, fibers, and proteins, respectively. The total number of occurrences of respective materials shows how many times that material had been brought up in literature to manufacture the bioplastics. Figure 4 represents researcher’sfrequently used (top 15, ranked according to their occurrences) materials. Analysis indicated that the bioplastics manufactured from polylactic acid are the most preferred, with the highest occurrences of 853, followed by starch and others (Fig. 4).
Polylactic acid (C3H4O2)n is an aliphatic thermoplastic polyester and non-toxic, compostable bioplasticmanufactured by polymerizing lactic acid monomers (C3H6O3) derived from the fermentation of starch. High molecular weight PLA (MW > 100000 D) possesses mechanical strength with a tensile strength comparable to conventional plastics that is 50–70 MPa.The degradation of PLA is influenced by pH and UV light; the alkaline condition favors PLA degradation [23]. PLA can be easily processed through film extrusion, blow or injection molding, fire spinning, etc.[24].
Starch is a biodegradable, cheap, renewable, and abundantly available natural biopolymer [25]. Various starch sources are corn, rice, wheat, potato, cassava, tapioca, sago, barley, jicama, oats, banana, date, soy, olive, peas, sugar, sugar beet pulp etc. High glass transition temperature and melting temperature of starch resist using starch itself as a bioplastic. However, blending with a plasticizer can increase the plasticity of starch [25]. A few authors[26, 27] stated that starch-based films become stronger, stiffer, and less flexible after several weeks of storage. Starch's retrogradation and moisture sensitivity are the major drawbacks but can be improved by blending with other biopolymers [2].
PHA (Polyhydroxyalkanoate) is a microbial thermoplastic polyester of R-hydroxyalkanoic acid. For the industrial production of PHA, sucrose and glucose are widely used due to their lower cost [28]. PHA with 3 to 6 hydroxyl acids holds physical,chemical, and mechanical properties similar to conventional plastics [29]. It possesses a higher melting temperature of PHA 168[30].Various bacteria derive PHB (Polyhydroxybutyrate) as an intracellular energy storage material. A biodegradable and biocompatible nature inveigles application in the packaging and biomedical field [31]. PHB holds upto 70% crystallinity and has good gas barrier properties[32]. However, several drawbacks, such as thermal instability, poor mechanical properties, and high production cost, can be overcome by blending PHB with other biopolymers. A few authors[24] suggested that mixing PHB with several monomers in different concentrations can regulate its thermal and mechanical properties
Cellulose is the most abundant natural biopolymer found mainly as the structural component of plant cell walls [33]. Cellulose is used in various forms such as cellulose acetate, carboxymethyl cellulose, cellulose ester, cellulose fibers, cellulose nanofiber, hemicellulose, lignocellulose, microcrystalline cellulose, nanocellulose, etc. Nanocellulose has a high specific surface area and has more hydroxyl groups and nanoscale morphology. It provides high strength, transiency, barrier property, low density, and low thermal expansion to bioplastics[34]. A few authors [35] have compared the biodegradability rate and strength of various cellulose forms which is provided below: Biodegradability rate: Hemicellulose >>> Accessible Crystalline Cellulose > Non crystalline Cellulose >>>> Crystalline Cellulose >>>>> Lignin; Strength: Crystalline Cellulose > > Non crystalline Cellulose + Hemicellulose + Lignin > Lignin
The stabilizers used are antimicrobial, antifungal, antibacterial, antioxidant, and flame retardants.Oils (like canola oil, castor oil, cooking oil, cottonseed oil, crude palm oil, edible oil, essential oil, fish oil, jatropha oil, kernel oil, linseed oil, olive oil, palm oil, rapeseed oil, soybean oil, vegetable oil, waste cooking oil)and metal oxides or nanoparticles (such as copper nanoparticles, silicon dioxide (SiO2), Titanium dioxide, TiO2 nanoparticles, calcium silicates, metallic nanoparticles, montmorillonite, pd alloy nanoparticles, nanoclay, oxide nanoparticles, silicate nanocomposites, zinc oxide nanoparticle, zinc oxide (ZnO), etc.) are being used. Chitosan has antibacterial properties that prevent the development of many fungi and bacteria on bioplastics’ surfaces. A few authors suggested that the chitosan Zn+and chitosan Ag+complexhavemore antimicrobial properties than chitosan[36, 37].
According to research done in[30], plasticizers' low molecular weight and non-volatile characteristics enhance bioplastics' flexibility. Plasticizers such as glycerol, sorbitol, chitosan, citric acid, and malic anhydride provide plasticity to bioplastics[24]. The study of[30] stated that the interaction of intermolecular hydrogen bonding could be reduced by blending glycerol with bioplastic, which offers crosslinking and improves the flexibility of bioplastics. Chitosan has low oxygen permeability, good film formability, and nontoxicity[38]. Because of its high antimicrobial activity, chitosan can also be used as an antimicrobial agent. A few authors [38] have found that sorbitol has increased the elongation with cellulose and chitosan and decreased the tensile strength.
Fibers reinforce bioplastics by enhancing strength and stiffness as they are lignocellulosic. The natural bio-fibers are leaf, bast, seed, and fruit. Carbon fiber, cellulose, nanofiber, wood fiber, and wool fiber are other fibers. The modulus of fiber increases with decreasing diameter of fiber[35]. Other properties such as density, tensile strength, mechanical strength, electrical conductivity, and moisture content depend on the fiber's molecular structure and chemical composition.Sources of proteins are beans, cheese whey, cotton-seed, gluten, keratin, silk, pea, rice, soy, whey, and by-products of the agri-food industries [39]. It has much lower mechanical properties [40]. However, it can be upgraded by chemical, enzymatic or physical protein treatments[41].
PHBV is a Polyhydroxybutyrate (PHB) derivative with good moulding properties, low crystallinity, low melting temperature, and less brittleness [10]. It has good mechanical properties and undergoes rapid enzymatic hydrolysis in sewage sludge, wastewater, and soil,but anaerobic hydrolysis is comparatively slow. Temperature, surface area, microbial density, composition, microbial infiltration, and enzyme percolation are several factors that influence the rate of degradation of PHBV [42].PCL (Polycaprolactone) is a thermoplastic polymer derived through the ring opening reaction of cyclic lactone monomer. PCLis a tough and semi-rigid material at room temperature, andthe modulus lies between LDPE and HDPE[35]. It provides good water, solvent, oil, and chlorine resistance [43]. According to the rheological study of PCL [44], it has higherelasticity and viscosity than PLA.
Various types of algae have been used for bioplastics. A few authors[45–48]havedefinedmicroalgae's bioactive composition as containing 12 to 48% lipids, 18 to 46% proteins, 18 to 46% carbohydrates, and 10 to 14% carotenoids. PVA (Polyvinyl alcohol) is a synthetic water-soluble polymer with good chemical resistance and biodegradability [37]. It is a hydrolysed product with admirable strength and flexibility [42]. PVA can resist oxygen permeation through bioplastic and has good mechanical, and thermal properties and transparency [49].Lower molecular weight PEG (Polyethylene glycol) improves elongation at break and softness while blending with other biopolymers[50].PEG can increase the mobility of PLA. PHV (Polyhydroxyvalerate) is a copolymer of PHB. The PHV has better physicochemical properties, such as lower crystallinity degree and more flexibility than PHB [51].
3.5 Most Productive, Collaborative, and Cited country, organization, author, journal, or publication in the field
The answer to the research question (Which country, organizations, authors, journals, publications are most impactful, productive, and collaborative?) is divided into five sections, (1) Most productive, collaborative, and cited country(2) Most productive, collaborative, and cited organisation (3) Most productive, collaborative and cited author(4) Most productive and impactful journal and(5) Most impactful publications.
3.5.1 Most productive, collaborative, and cited country
The co-authorship analysis in the country’s unit represents the productive country with the most published documents. A country's total link strength in that network represents its collaborative nature, which means higher link strength between two countries indicates strong collaborations. The total link strength of a country depends upon the minimum number of documents and citations. Here, a minimum of 3 documents and 3 citations from each country were taken to generate the network. Thus, of the 99 countries, 72 countries meet the threshold. Among them, the largest set of connected countries consists of 71 countries. The bibliographic map contains 10 clusters with 71 countries, 479 links, and 1,299 total link strength, and it shows that the USA is the leading country with 685 published documents, 22,913 citations, and 247 total link strength. Japan follows the listwith 387 publications, 7,873 citations, and 115 total link strengths. Italy was positionedin3rd place with 180 total link strengths in the most collaborative country. India ranked sixth with 242 publications and is the tenth most collaborative country. However, India ranked fifth in the most cited country with 6597 citations. The bibliometric map (Supplementary, Annexure – D, Fig. D.5) presents the network of the most productive country in the field.
The analysis of the most cited countries can be analysed by the citation analysis in the country's unit. The link strength between two countries in the network represents the number of times two countries have been cited together. Here, the map is generated by taking a minimum of 5 documents and 5 citations of a country. Thus, of the 99 countries, 60 countries meet the threshold, and the largest set of connected countries consists of 60 countries. The following map (Supplementary, Annexure – D, Fig. D.5) consists of 9 clusters with 60 countries, 1,107 links, and 13,803 total link strength. The map represents the USA's most cited country with 22,913 citations and 3,346 total link strengths. The bibliometric map (Supplementary, Annexure D, Fig. D.6) represents the network of the most cited country in the field. Figure 8represents the top sixteen productive, cited, and collaborative countries.TheUSA has produced the highest number of documents with high collaboration and citations.
3.5.2 Most productive, collaborative, and cited organisation
The co-authorship analysis technique in the organization unit represents the most productive organisations by the number of publications published in the particular field. In the bibliometric map, the link strength between two organisations represents the number of collaborations between those two organisations. Here, the network is created by taking a minimum of 5 documents and 5 citations of an organization. Thus, of the 2,921 organizations, 308 organisations meet the threshold, and the largest set of affiliated organisations consists of 281. The presented network (Supplementary, Annexure D, Fig. D.7) consists of 22 clusters with 281 organisations, 666 links, and 1134 total link strength showing the most productive institutions in the field. The Washington state university is the leading organisation by publishing the highest 77 number of documents and the leading collaborative institute with a total links strength of 82. The University of Seville stands in the second position with 54 published documents.
The citation analysis in the organization unit is used to analyse the most cited institutions in the field. It represents the most cited organisations with the number of citations in a particular field. The total number of times the two organisations have been cited together is represented by the link strength between the two organisations in that network. Here, the network is created by taking a minimum 10 number of documents of an organization and a minimum 10 number of citations of an organization. Thus, of the 2,921 organizations, 107 organisations meet the threshold, and the largest set of affiliated organisations consists of 106 organisations. The bibliometric map (Supplementary, Annexure D, Fig. D.8) consists of 8 clusters with 106 organisations, 1,274 links, and 4,311 total link strength, representing the network of the most cited institutions. The analysis shows that Michigan state university has the highest number of citations which is 5,538. Then CSIR has 2,396 citations, and the University of Guelph has 1563 citations. Figure 9 represents the top sixteen productive, collaborative, and cited organisations in the field of bioplastics.Michigan State University has produced a maximum number of documents andhas higher citations, but its collaborations are not as good as that of Washington State University, which has the highest collaborations.
3.5.3 Most productive, collaborative, and cited author
The author's unit can analyze the most productive and collaborative author through co-authorship analysis. The network represents the most productive author by contributing to the highest number of publications. The link strength between the two authors in that network represents their collaboration in various documents. Here, to generate the map, a minimum 3 number of documents and three citations of an author were taken. Thus, of the 12,220 authors, 835 meet the threshold, and the largest set of connected authors consists of 118. The bibliometric map (Supplementary, Annexure D, Fig. D.9) consists of 13 clusters with 118 authors, 355 links, and 912 total link strengths, representing the network of the most productive authors in the field. From the findings, ManjusriMisra has published the highest 38 documents with 1,413 citations and has 56 total link strengths. Amar Mohanty follows that with 35 publications, and Kumar Sudesh with 25 publications in the field. The finding showsthat Takashi Watanabe is the most collaborative author, with 114 total link strengths.
The citation analysis in the author's unit represents the most cited author in the field. The link strength between two authors in that network represents the number of times that two authors have cited together. Here, to generate the network, a minimum 5 number of documents and five citations of an author have been taken. Thus, of the 12,220 authors, 280 authors meet the threshold, and the largest set of connected authors consists of 268. The bibliometric map (Supplementary, Annexure D, Fig. D.10) consists of 14 clusters with 268 authors, 2429 links, and 16918 total link strength representing the network of the most cited authors in the field. Amar K. Mohanty is the most cited author with 4755 citations. Then, M Misra has 4582 citations, and Lt Drzal has 2761 citations. Figure 10 shows the field's most (top sixteen) productive, collaborative, and cited authors.Amar K. Mohanty and ManjusriMisra produced the highest number of documents and have higher citations and collaborations.
3.5.4 Most productive and impactful journal
The citation analysis in the source unit represents the journals that the researchers cited. By changing weights to documents, we can figure out the number of publications per journal in a particular field. In the network, the link strength between two journals represents that two journals have been cited together. A minimum 5 number of documents of a source and 5 citations of a source were considered to generate the bibliometric map. Thus, of the 1,106 sources, 156 meet the threshold, and the largest set of related items consists of 155 items. The bibliometric map (Supplementary, Annexure D, Fig. D.11) consists of 10 clusters with 155 sources, 2,203 links, and 5,403 total link strength. As per the outcomes, the Journal of Polymers and the Environment is the most productive as it contains 121 documents in the field. The Journal of Applied Polymer Science ranked second with 106 publications. Bioresource Technology is the most-cited journal, with 4,172 citations, followed by the Journal of Polymers and the Environment with 3,594 citations. The bibliometric map (Supplementary, Annexure D, Fig. D.11, D.12) shows the network of the most productive and most-cited journals in the field, respectively. The following Fig. 11 represents the most (top ten) productive & cited journals.
3.5.5 Most impactful publications
To analyse the most impactful publication of the field, the citation analysis in the unit of documents can be used. The bibliometric network of this analysis represents the most cited documents in the particular field, and the link strength between two documents represents the number of times that two documents have been cited together. Here, the network was created by taking a minimum 70 number of document citations. Thus, of the 3,802 documents, 233 meet the threshold, and the largest set of related documents consists of 191 documents. The bibliometric map (Supplementary, Annexure D, Fig. D.13) consists of 20 clusters with 191 documents and 430 links. The most impactful publication in the field is “Biofibers, biodegradable polymers, and biocomposites: An overview,” with 1993 citations authored by A. K. Mohanty, M. Misra, and G. Hinrichsen and published in the Macromolecular Materials and Engineering journal. Table – 1 shows the top 10 impactful publications in the field.
Table 1
Most impactful publications
Rank | Document Title | Citations | Reference |
1 | Biofibres, Biodegradable Polymers, and Biocomposites: An Overview | 1993 | [35] |
2 | An Overview of The Recent Developments in Polylactide (PLA) Research | 1317 | [52] |
3 | Sustainable Biocomposites from Renewable Resources: Opportunities and Challenges in The Green Materials World | 1288 | [53] |
4 | Biological Degradation of Plastics: A Comprehensive Review | 873 | [54] |
5 | Biodegradable Polymers for Food Packaging: A Review | 868 | [55] |
6 | A Microbial Polyhydroxyalkanoates (PHA) Based Bio and Materials Industry | 697 | [56] |
7 | Biodegradable Composites Based on LignocellulosicFibers – An Overview | 690 | [57] |
8 | Advances in Cellulose Ester Performance and Application | 680 | [26] |
9 | Surface modifications of natural fibers and performance of the resulting biocomposites: An overview | 556 | [58] |
10 | Thermoplastic Films from Plant Proteins | 505 | [59] |