et al. 2018) or with products that are rendered photodegradable or enhanced biodegradable (a 0.5 wt% of biodegradable PLA contaminating a PET recycling stream discolors the rPET) (Alaerts et al. 2018). Some of waste plastics, however, may be amenable to recycling into chemical feedstock and energy via either pyrolysis or incineration. Recycling of waste fishing gear into plastic resin pellets, and their conversion to energy by incineration, are practiced at a small scale in the US, but much progress needs to be made.
Recycling does involve significant costs for collection, sorting, classifying, and cleaning of waste plastics, and there are incidental losses of material during the process that preclude a 100% mass recovery as the recyclate. Even with such losses, the savings in energy are substantial. There are, however, embedded impacts associated with the recycling process as well, and LCA must be carried out to make sure that most of what is saved in energy is not negated by increased environmental impacts. Also, each time a resin is recycled its mechanical characteristics detriorate; with PET, 3–5 repeated recycling results in ~50% decrease in tensile strength and extensibility of the resin (La Mantia and Vinci 1994).
1.4.2 Using Bio‐Based Feedstocks for Plastics.
Plastics can be classified based on the feedstock used in their synthesis, into those that are fossil fuel‐based and those derived from biological resources. Conventional plastics are mostly synthesized using feedstock derived from fossil fuel while the second category of plastics is synthesized using plant biomass as feedstock, and ISO, as well as the ASTM, refer to these as “bio‐based” plastics. The carbon in the plant biomass is derived from the present‐day atmosphere or the carbon cycle. However, it is important to recognize that “bio‐based” plastics defined in this manner include two distinct categories of plastics; those made from monomers derived from biomass and polymers synthesized by living organisms (or biopolymers) that are modified by man. Thus, three broad classes of biologically derives plastics might be identified.
1 Biopolymers that are synthesized by living organisms and exist as polymers in the biomass, including cellulose and poly (hydroxyl alkanoates) [PHAs]. These are extracted and used with no further chemical modification of the polymer.
2 Modified biopolymers where a biopolymer is extracted from biomass and chemically changed prior to use, as in the case of cellophane. Dissolving plant cellulose in carbon disulfide (in the xanthate process) or copper salt/ammonia (in viscose process), and re‐precipitating the solution in dilute acid yields cellophane and rayon. The material is still plant‐derived cellulose but with an altered secondary and tertiary structure of the cellulose molecule. Similarly, in the alkaline de‐amidation of chitin 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