from crab shells to obtain chitosan, the primary structure of the polymer is changed with the substitution of the amide with an amine group.3 Bio‐based plastics use plant‐based biomass feedstock to synthesize monomers that are polymerized into plastics such as PE or PP. For instance, the sugars in waste sugarcane or sugar‐beet residue are fermented into alcohol that is easily converted into ethylene, a monomer used to synthesize Bio‐PE or PLA. Using renewable bio‐based feedstocks can help conserve fossil fuel reserves.
The Bio‐PE for instance, is identical in its properties to the conventional PE made from fossil fuel, except that it is “bio‐based.” Bio‐PE, Bio‐PP, and Bio‐PET all have processing characteristics identical to their conventional counterparts, allowing easy substitution (or drop‐in) in standard processing operations practiced in the plastics industry. Bio‐based resin and conventioniral resin (from fossil fuel) are chemically indistinguishable except for the isotopic ratio of the carbon in the molecules or the (13C:12C) or (∂13C) (Suzuki et al. 2010). Measured (∂13C) values of a polymer reveal the lineage of its carbon atoms, distinguishing between those derived from renewable biomass and those from fossil‐fuel resources. The carbon in the latter is ancient, having formed millions of years back in time, while that in biomass carbon is derived recently from the CO2 in today’s atmosphere, accounting for this difference in their isotopic ratio. Most of the carbon in chemically modified biopolymers is also derived from the atmosphere.
With polymers synthesized by polycondensation of two monomers, one monomer can be bio‐based while the other is derived from fossil‐fuel feedstock, leading to a hybrid or a partially bio‐based plastic. This is the case with hybrid poly(ethylene terephthalate) (PET) resin that is popularly used in “green” beverage bottles in the market, that are only about 22% bio‐based; the ethylene glycol monomer is bio‐based while terephthalic acid is derived from fossil fuel (Figure 1.8).
There is confusion in the literature as to how the environmental biodegradability of plastics might relate to the above categorization. The biodegradability of plastics in a biotic environment is determined by their chemical structure; the polymer molecule must have main‐chain bonds that are hydrolyzable by enzymes secreted by the microorganisms in the relevant environment. There is no relationship between the source of feedstock and the biodegradability of the resin, as seen from Table 1.3 and Figure 1.9. Biopolymers such as cellulose or chitin have been in the environment for a very long time allowing biochemical pathways that degrade these to evolve and therefore they tend to be biodegradable. This is not the case with synthetic man‐nade plastics that have existed in the environment only since the beginning of the anthropocene. Some authors (Brizga et al. 2020) confusingly include blends of a synthetic polymer with a degradable additive such as starch under “biodegradable” plastics. In these materials such as blends of starch/PE, the polymer component does not biodegrade appreciably.
Figure 1.8 Hybrid PET with ~23% of bio‐based content (a fifth of carbon atoms is from biomass).
Table 1.3 A simple classification of plastics based on their feedstock.
| Category | Criterion | Example |
|---|---|---|
| Fossil‐fuel based polymers | Man‐made polymers made from monomers derived from fossil‐fuels | PE, PP, PS |
| Biomass‐based polymers | Man‐made polymers are derived from monomers derived from biomass. | PE, polyurethane, PLA |
| Biopolymers synthesized by a living organism | Polymers are synthesized by a living organism. | Cellulose, Chitin |
| Structurally modified biopolymers | Biopolymers that are chemically altered to improve properties | Rayon, cellulose acetate, chitosan |
As already pointed out, increases in plastic production will further deplete fossil fuel reserves and be accompanied by significant emissions, especially CO2, into the atmosphere (Spierling et al. 2018). Both these negative environmental impacts might be reduced to some extent by using more bio‐based plastics (Narancic et al. 2020; Zhu et al. 2016). Using biodegradable plastics made of bio‐feedstocks will also help in waste management (Calabrò and Grosso 2018), especially in the marine environment. Presently, their annual supply is limited with only 2.1 MMT (2019 data) of bio‐based resins accounting for only ~1% of total global plastic production. Over 57% of their production was PLA and PBAT (poly [butylene adipate‐co‐terephthalate]), the highest volume biodegradable resin manufactured. In the bio‐based category, PE, Nylon, and PET were the highest‐volume resins manufactured.6
Figure 1.9 Classification of plastics according to inherent biodegradability.
In evaluating the merits of substituting conventional petrochemicals with bio‐based feedstocks, close attention must be paid to the scope of the relevant life‐cycle analyses (LCA). Bishop (2020), in a review of 44 studies comparing bio‐based with conventional feedstock for plastics, found that 84% of them did not account for additives in the inventories used in their analyses and most did not adopt a broad enough domain of impacts. For instance, using either sugar beet or wheat, yields a biomass yield of 73 T/ha and 8.6 T/ha, respectively, but the crop needed per ton of PE was 23.9 T and 6.84 T, respectively. On an area cultivated basis, sugar beet yielded a benefit in reducing climate change, that was at least two times greater than that by wheat bran biomass, underling the complexities of selecting the proper biomass source for feedstock (Belboom and Léonard 2016). The bio‐based and biodegradable plastic PLA, is often used in food‐service items. A “cradle‐to‐grave” comparison of PLA, PP, and PET for the fabrication of beverage cups found that PLA was superior (by 40–50%) to PP and PET, both in terms of climate change impacts as well as fossil fuel conservation (Moretti et al. 2021). But, in other impact categories such as eutrophication, acidification, particulates, and photochemical ozone formation, PLA was found to be worse than both conventional plastics. Mainly due to the lack of harmonization, it is difficult to compare different LCA studies on biobased resins (Cheroennet et al. 2017; Papong et al. 2014; Simon et al. 2016) with each other. Spierling et al. (2018), in their review of LCA on bio‐based plastics, concluded them to significantly contribute to environmental sustainability, potentially eliminating the emission of 240–315 MMT of carbon equivalents at a 65.8% substitution of conventional plastics in use. As with using any material in a given application, there are environmental and economic trade‐offs to be considered when using bio‐based resins as well. A full environmental assessment of the candidate bio‐plastic, based on LCA for the particular application, is a prerequisite for their adoption.
1.5 Plastic Manufacturing
Plastics manufacturing is too broad a subject
