Can plastic also be used as fertilizer? Bioplastics have made it possible

During the oil boom of the 1950s, chemists began to convert refinery waste into plastic products - plastic packaging, plastic furniture, and plastic fibers that

could be woven into synthetic fabrics. These materials have plasticity, flexibility, and durability, and are hailed as a great invention. In the following decades,

global plastic production surged: humans have cumulatively produced 8 billion tons of plastic products.

To put it mildly, this plastic craze has also brought many problems. More than half of the world's plastic products (approximately 5 billion tons) are scattered on

the surface in the form of fragments. More than 10000 tons of plastic waste enter the ocean every day. The miraculous durability of plastic has enabled it, but

now it has become a stubborn source of pollution.

However, to be fair, plastic has indeed changed the world. From cars, mobile phones to computers, many important technologies rely on plastic materials.

Foamed plastic heat insulation material has increased the energy efficiency of the house by 200 times, and plastic film has significantly extended the shelf life

of perishable food.

Physicist Eleftheria Roumeli from the University of Washington pointed out in the 2023 Materials Research Annual Review that demonizing plastic as the worst

invention of humanity is unfair, and it is actually the crystallization of engineering wisdom

She believes that instead of completely abandoning plastic, we should look for a better and more friendly alternative - one that maintains the ductility and

flexibility of modern plastics, uses sustainable biological materials, and achieves environmentally friendly degradation. This means that we need to thoroughly

rethink the way we produce plastic.

In 2019, an experimental research collaboration project called Korvaa successfully developed the world's first microbial cultivation headset. The device consists of components in different forms such as rigid structure, foam buffer layer and fabric covering layer, and all materials are prepared from bio based raw materials. Image source: AIVAN

From monomers to polymers

The current plastic production method consists of two main steps: decomposition (cracking) and then reassembly (polymerization).

The decomposition process, also known as "cracking," is carried out at high temperature and high pressure to convert refined petroleum raw materials

into simple molecules called monomers. These molecules form the backbone of the recombinant product. The chain or mesh structure generated is

called a polymer, which constitutes the basic structural components of all plastics.

But plastic production has not yet been completed. Next, additives such as coloring agents, flame retardants, and fillers need to be added. Materials

scientists need to consider multiple variables, from "hardness" to "tear strength" to "tensile modulus", which reflect the performance of plastics under

different stresses. The most critical additive regulates these properties by adjusting the intermolecular bonding of polymer chains. For example, chemicals

called plasticizers are embedded between chains to enhance flexibility, but at the cost of making the plastic more prone to tearing.

By blending polymers and additives, chemists have finally made composite materials for food packaging films, beverage bottles, cosmetic beads,

and even as flexible hydrogels - such as contact lenses attached to the cornea - to correct vision. Through chemical means, a single polymer such

as polyvinyl chloride (PVC) can be used to make both rigid rainwater pipes and clothing.

Researchers are exploring how to produce monomers and polymers from biomass, as well as how to use intact organisms and tissues as raw materials.

Monomers and other small molecule raw material units require more processing, but are easier to use in existing production facilities. Plastic production

accounts for 8% of global fossil fuel consumption - it is estimated that this proportion may rise to 20% by 2050. But decades before the rise of the

petroleum industry, chemists had already made "synthetic" plastics from materials such as oat shell waste and vegetable oil. One of the paths to

achieving more sustainable plastics is to return to such biogenic materials.

For example, in 2006, Braskem, a Brazilian petrochemical company, launched experiments to study whether sugar could be economically

converted into ethylene, the most important monomer in the production of bulk plastics. By 2010, Braskem began selling "biobased" polyethylene

plastics (hereafter referred to as biobased PE).

The biggest advantage of this material is that sugarcane can fix carbon in the atmosphere during its growth. Due to its structural similarity to

chemically synthesized PE, bio based PE is easily applicable in fields such as food packaging, cosmetics, and toys. But the same chemical structure

also brings problems. Due to the absence of polyethylene in the natural environment, few microorganisms have evolved the ability to break down its

molecular bonds. Therefore, bio based PE cannot solve the waste problem. In other words, this' bioplastic 'does not necessarily imply inherent sustainability.

These terms lack sufficient regulation and clear definitions, leading to a lot of confusion, "said Rachel Meidl, an energy and sustainable development

researcher at the Baker Institute at Rice University.

Meidl divides plastics and their alternatives into four quadrants: one axis represents the material source (biobased/petroleum based), and the other

axis represents the downstream destination (biodegradable/non biodegradable). But even materials that are in the best quadrant - both bio based

and biodegradable - may not necessarily be a panacea. 'Biodegradable' only refers to materials that can be decomposed by microorganisms, even

if the result is the production of microplastic fragments. The ideal material not only needs to have biodegradable properties, but also needs to be

compostable - a higher requirement material that can degrade into organic components that are harmless to animals and plants.

Unfortunately, compostability is difficult to achieve. Everyone must have come into contact with compostable tableware and takeaway lunch boxes

made of polylactic acid (PLA). As the most common biobased plastic, PLA is theoretically compostable, but requires specific industrial facility

conditions, and the current number of such facilities is insufficient. Due to the current situation where PLA meal boxes are often mixed with kitchen

waste, composters have to spend time sorting the two.

One way to improve plastics is to search for better bio based monomers. In 2020, the team of California scientists announced that the polyol

monomer was separated from the algal oil, and then reorganized into foam plastic that can be used for commercial footwear. This material

can effectively degrade in soil. However, some scientists believe that the high energy consuming two-step standard process of "decomposition

recombination" should be abandoned. David Kaplan, a biomedical engineer at Tufts University, pointed out that nature already provides promising

compostable polymers. Due to their different degradation time scales, selecting appropriate polymers or regulating them can produce materials

that are suitable for different applications.

Taking cellulose as an example - the most common biopolymer in plant cell walls. Its essence is sugar molecule chains, but these chains will

form nanofibers, which then aggregate into microfibers, ultimately forming visible large fibers (such as the filamentous structure in celery).

Materials scientists refer to this as hierarchical structure.

In contrast, synthetic polymers are typically extruded into homogeneous masses through funnel compression. The result is the formation

of "strong and hard bonds" between molecules, "Kaplan said. This phenomenon is rare in biology. On the contrary, the bonds in biopolymers

are much weaker - usually electrostatic interactions that connect hydrogen atoms in one polymer molecule to hydrogen atoms in another

molecule, but the density of these interactions is very high

However, with a deeper understanding of these structures, engineers can improve biomaterials. Research has shown that finer cellulose fibers

have higher tensile strength, meaning they are more resistant to fracture under tension. The increase in surface area makes it easier for

hydrogen atoms to dynamically establish/break bonds between adjacent chains.

Cell level utilization strategy

Since we have abandoned the use of monomers and eliminated a whole link in plastic production, why not go further? Some materials scientists are

practicing Kaplan's "bottom-up design": directly using whole cells or other biological materials to manufacture bioplastics without the need for

decomposition and extraction.

For example, Roumeli has developed the potential of algae cells: they are small in size, easy to manipulate, and rich in proteins (biopolymers)

and other useful substances. She and her team processed the algae powder through a hot press. After multiple experiments, by adjusting the

hot pressing time, temperature, and pressure (all of which affect the molecular bonding method), a material with better strength than bulk

commodity plastics was finally produced.

This material can also be recycled: it can be ground into powder and pressed into shape again. (Tests have shown that the strength decreases

slightly after each recycling, and the same goes for synthetic plastics.) If casually discarded in soil, its decomposition rate is comparable to that

of banana peels.

Researchers at the University of Washington have hot pressed blue-green algae (spirulina) cells into bioplastics, which are stronger than most

application requirements and can be composted at home.

Kaplan conducted similar research in the field of silk - traditionally, silk is considered fragile and not heat-resistant, and there is concern that

hydrogen bonds may break due to heat, leading to carbonization or even combustion. However, in 2020, the Kaplan team demonstrated that

silk particles can be injection molded like plastic, and since then, they have discovered that entire cocoons can also be processed in this way.

Roumeli claims that this type of material is a win-win situation: renewable, fossil fuel free, carbon sequestration during growth, and completely

biodegradable. The only disadvantage is economy and scalability. She said.

Perhaps the biggest pain point of the new method lies in its innovation, which leads to high current costs. Reducing costs requires utilizing existing

production facilities to avoid start-up companies bearing huge equipment investments. Chemist Gadi Rothenberg from the University of Amsterdam

pointed out that existing factory owners may consider biocomposite materials too impure or even "garbage".

Rosenberg pointed out that the raw materials used to produce polyethylene terephthalate (PET), which is the plastic used to make soda bottles,

contain only one non target monomer per 100000 molecules. Biological materials rarely have such purity.

Manufacturers tend to choose mature solutions rather than innovative materials. Rothenberg develops plant-based polymers, which he believes

can seamlessly replace furniture materials. But when he first brought this material to the company, "initially the company didn't want to know," he

admitted. Data shows that the production cost of chemically equivalent sugarcane based PE is 30% higher, and profit oriented companies still

choose traditional products.

Currently, data from the European Plastics Association shows that the market share of bio based plastics is less than 1%. Rothenberg emphasized

that bio based polymers need to achieve economic parity in order to have prospects. He believes that the government needs to calculate the true

cost of traditional plastics (carbon footprint+pollution control) before sustainable materials can be widely adopted.

But leading scientists remain optimistic, as Roumeli points out that "the cheapest, most produced, and most consumed plastic today" - once a novelty.

Kaplan firmly believes that "all these precursors and polymers will be manufactured through biological means, or with true recyclability as a starting point

But we haven't reached that point yet, "he added. The problem is that plastic pollution and climate change leave little time for humanity.