The Ultimate Guide to Plastics of the Future Part 11 – Plastics Innovation = Materials Innovation

Plastic has been the most widely-used Material in manufacturing for the last 70 years. In fact estimates show that we have produced nearly 10 billion tonnes of plastics since 1950.

We see it all around us, and not simply durable goods. Many plastics are used for packaging (plastic’s #1 use!) and other single-use materials. These single-use materials are ever increasing in production, and we only have to look at the last few years of the COVID-19 pandemic to see the rampant rise in gloves, masks, disposable test kits and other medical products which all contribute to single-use waste, and are rarely recycled.

While plastic has many advantageous qualities, it also poses tremendous environmental challenges. Plastic production emits greenhouse gasses, and wastewater from plastic manufacturing can pollute waterways and oceans. Not to mention the fact that plastic can take decades, or centuries to break down, meaning the world is filling up with plastic waste at an alarming rate.

Each year we dump nearly 50 million tonnes of plastic on the Earth and in its oceans, and that rate is expected to double by 2025! Our economic successes with plastics have come at a huge cost to our planet.

Plastics are a major contributor to the world’s carbon footprint and the industry is now looking for ways to reduce that.

With so much of the world’s products containing plastics, the mere innovation of polymers will carry over to virtually all manufacturing sectors and affect the path to sustainability for a majority of the products we use. Materials innovation will be on many fronts, but just the sheer number of patents applied for in the last decade, shows a clear trend of plastics innovations leading the way.

There are two tracks of innovation that will lead to sustainable, circular plastics – 

1. Recycling 
2. Alternative Materials

Traditional mechanical recycling (which we touched on earlier in this article) is an old process that takes time, uses a lot of energy, and is very labor intensive. There will be innovations around mechanical recycling, but primarily in the “preparation” of plastic materials to be recycled – collecting, sorting, washing, and the breakdown process to convert the plastics back to their original resin state (be it mechanical or chemical).

Chemical recycling will see more innovative concepts over the coming years than mechanical ones, as those processes have more promise in breaking down polymers closer to their original state, meaning the feedstock will be “cleaner” than what is produced through mechanical recycling. 

Alternative materials for plastics and plastics additives will see the greatest rate of innovation in the foreseeable future. There will be new formulations that are easier to produce, new production methods that cut the time and energy needed to manufacture, and new raw materials used that will reduce the overall carbon footprint of the plastics we use every day.

Innovations in Recycling

Most of us think of plastics recycling as simply grinding down and melting plastics to simply reuse the material and mold into new products. While that is a piece of the cycle, there’s much more to recycling. There’s no single solution to recycling as each polymer requires different methods and sorting plastics is a tough task – think about recycling a multilayer laminate!

There are 6 major elements to the recycling puzzle:

1. Pre-consumer plastic to product
2. Post-consumer plastic to product (what is mentioned above)
3. Plastic to feedstock
4. Plastic to compost
5. Plastic to monomer
6. Plastic to incineration for energy recovery

Mechanical Recycling

Mechanical recycling (heat melt, grind, etc.) is the most common type of plastics recycling in use today. It uses established technologies and can be very labor intensive, especially when using post-consumer materials for the feedstock, as this is basically our waste material collected in various forms and conditions.

Post-industrial recycling is much easier, however this is typically done within a plastic’s manufacturing facility and using the “clean” scrap materials discarded during molding. Thus, there is much less feedstock available, as manufacturers put measures in place to reduce scrap in the first place.

Post-consumer mechanical recycling has other limitations, such as collection, sorting and the fact that the material can not be completely restored to its virgin state easily, and that the polymers suffer degradation during the process. This limits the use cases for mechanically recycled feedstock, and points to the uptick in innovations regarding chemical and biological recycling.

Mechanical recycling, while the least expensive and most common, still has inefficiencies built in. During the process of sorting, washing, grinding and melting, not all contaminants get removed. Also, there is great stress on the polymer under heat. This means that the output of plastic products are almost always downgraded to goods of lesser quality than the virgin materials.

To reap the full rewards of plastics recycling, the processes may need to shift from a mechanical to chemical process.

Chemical and Biological Recycling (Will Lead the Way)

Researchers from the University of Delaware’s Center for Plastics Innovation (CPI) have developed a direct method to convert single-use plastic waste — plastic bags, yogurt containers, plastic bottles and bottle caps, packaging and more — to ready-to-use molecules for jet fuels, diesel and lubricants.

The work is led by Dionisios Vlachos, Unidel Dan Rich Chair in Energy and Professor of Chemical and Biomolecular Engineering. Vlachos’ research focuses on circular economy and waste derivatization, multiscale modeling and simulation, distributed (bio)chemical manufacturing,

He focuses on using a novel catalyst and unique process to quickly break down these hardest-to-recycle plastics, known as polyolefins. Polyolefins account for 60 to 70% of all plastics made today. The catalyst itself is actually a hybrid material, a combination of zeolites and mixed metal oxides.

Zeolites are known to have properties that make them good at creating branched molecules. Zeolites are found in things like water purification or softener systems and home detergents, where they counteract minerals like calcium and magnesium, making hard water softer and improving the laundry process.

Mixed metal oxides, meanwhile, are known for their ability to break down large molecules at just the right amount without overdoing it. The antacid in your medicine cabinet, for example, is a metal oxide used to break down, or neutralize, the acid causing your upset stomach.

“Alone these two catalysts do poorly. Together, the combination does magic, melting the plastics down and leaving no plastic behind,” Vlachos said. “This makes them ready-to-use molecules for high-value lubricant or fuel applications.”

Biomass

Lignocellulosic biomass is the most abundant renewable carbon source on Earth.  Available biomass sources include forest residues, crop residues, purpose-grown energy crops (e.g. grasses), animal wastes and food wastes.  These materials are the fibrous structural parts of plants and are largely made of cellulose, hemicellulose and lignin.  

Compared with so-called 1st generation bio-feedstocks such as sugars, starches and vegetable oils, nature has made these parts of the plants difficult to deconstruct to chemical building blocks, making utilization of this carbon source a challenge for scientists and engineers.   Biorefineries are facilities where biomass is converted to a variety of products.  

Target products included advanced hydrocarbon biofuels that are indistinguishable from fossil-based gasoline, diesel or jet fuels along with bio-based chemicals and materials.  Technologies need to be developed to more efficiently convert this renewable carbon source so that renewable bio-products from biomass can be made economically competitive with those produced from fossil resources.

Pyrolysis

Pyrolysis is one of the technologies available to convert biomass to an intermediate liquid product that can be refined to drop-in hydrocarbon biofuels, oxygenated fuel additives and petrochemical replacements. Pyrolysis is the heating of an organic material, such as biomass, in the absence of oxygen.  Biomass pyrolysis is usually conducted at or above 500 °C, providing enough heat to deconstruct the strong bio-polymers mentioned above.   

Because no oxygen is present combustion does not occur, rather the biomass thermally decomposes into combustible gasses and bio-char.  Most of these combustible gasses can be condensed into a combustible liquid, called pyrolysis oil (bio-oil), though there are some permanent gasses (CO­2, CO, H2, light hydrocarbons), some of which can be combusted to provide the heat for the process.  

What is the Difference Between Pyrolysis and Gasification?

Pyrolysis is the process of thermal conversion of organic matter using a catalyst in the absence of oxygen. Gasification is a thermo-chemical process that converts biomass into a combustible gas called producer gas (syngas). The key difference between pyrolysis and gasification is that pyrolysis is done in the absence of air while gasification is done in the presence of air. Besides, the products of pyrolysis are heat and combustible liquid and combustible gas while the products of gasification include heat and combustible gas. So, this is also a difference between pyrolysis and gasification.

Moreover, pyrolysis is useful for applications in food manufacturing, i.e. caramelization, production of fuel from biomass, production of ethylene, to treat plastic waste, etc. while gasification is useful for heat production, production of electricity, etc.

Plastic to Monomer – How does it work?

‘Depolymerization’ is one of the ways to chemically recycle plastic waste. In this process, sorted plastic waste is broken down into monomers (basic building blocks) to feed them back into the plastic production.

The depolymerization recycling process starts with an initial step where plastic waste is sorted and prepared for further processing. The depolymerization process – often referred to as chemolysis or solvolysis – uses different combinations of chemistry, solvents and heat to break down polymers into monomers. 

Monomers are the building blocks of polymers. In the following step, potential contaminants are isolated from the monomers to remove them. The monomers are then fed back into the normal plastic production processes as a secondary raw material. The plastics produced this way are of similar quality than those made from traditional fossil resources.

Finally, there are new methods of reducing polymers to their original state and the goal of returning 100% of the materials to their virgin state may not be too far off in the future. PureCycle Technologies is a recycler that is using a new patented process developed by Procter and Gamble to bring polypropylene back to its original form, by effectively removing all the colorings, odor and other contaminants from the post-consumer feedstock. PureCycle announced earlier this year that they now use less energy to recycle polymers than what is needed to manufacture new feedstock. This could open the door for price parity for recycled materials. 

Innovations in Alternative Materials

With 90% of all raw plastics coming from fossil feedstock (either oil or natural gas) it’s obvious that better sources of raw materials need to be found. We know that we cannot simply stop manufacturing plastics, they are too ingrained into our raw materials supply chain. So, we must find alternative materials that do not come with the same carbon burden of fossil fuels.

Alternative materials will negate the bad effects of traditional plastics four ways:

1. Increased Reusability
2. More Efficient Recyclability
3. Higher Biodegradability – Biological Composting
4. Lower Carbon Footprint Feedstock

Reusability

Reusing materials is not a new idea. Before the rise of inexpensive plastics, most of our packaging items were reused – think milk/soda bottles, burlap shopping bags and tin boxes. All of these items were more expensive to manufacture and many regions placed value on them to encourage recycling (long before it was fashionable to do so).

We brought bottles back to stores, tins were collected by potato chip sales people and we went to the corner market with sacks from home to carry our groceries.

However, with the advent of plastics, packaging materials became more “disposable”. One reason was the low cost, the other was the sheer volume of material. A milk bottle made of glass had far more weight and volume than a plastic jug, so it felt more acceptable in our minds to simply throw them away. After all, they didn’t seem as robust as glass, and most likely wouldn’t make it through a general reuse cycle without damage.

Today, manufacturers (like Unilever and Procter & Gamble) are looking at more substantial plastic packaging to encourage many cycles of use, refill and reuse.

More Efficient Recyclability

New plastic materials are being commercialized that are easier to recycle. It’s not just a matter of improving the mechanical or chemical process of recycling plastics, but also creating plastics that are easier to break down into raw polymers.

New materials like vitrimers, will play a big role in reusability.

While some polymers are very rigid, strong and work well in structural applications; there are others which are more quickly manufactured and easier to mold and to recycle. The tradeoff has been that if it’s easy to manufacture it will in turn lack strength, rigidity and tolerance to high temperatures. If the composite has exceptional strength and durability, then it is much more difficult to manufacture.

Until recently synthetic polymers have resided on these two sides of usability:

1. Thermosets – a group of polymer materials that have exceptional mechanical properties. They have a very low coefficient of thermal expansion, good chemical durability and are very resistant to fatigue.
2. Thermoplastics – the type of polymer material that can be melted down and reprocessed, remolded and reused. They are ideal for quick, high-volume manufacturing such as injection molding. Thermoplastics are also ideal for recycling as they can be melt-processed.

Vitrimers are processed as covalently-bonded crosslinked network polymers, however their chemical bonds are exchangeable. Meaning that with sufficient heat, the bond exchange increases, but the crosslink density remains the same.

Ultimately, they can cure without the need for catalysts, and virtually no limit on how many times the material can be heated and cured. It becomes malleable only above glass transition temps, so this means below those temperatures, the polymer acts exactly like a traditional thermoset with the monomers “frozen” in place, making it almost indefinitely reusable and recyclable.

Biodegradable and Compostable Plastics

As we mentioned earlier, single-use plastics are a key contributor to the sheer amount of plastics piling up in landfills and our oceans. These end-of-life plastics are amassing in landfills resulting in both management issues and environmental harm and will take decades or more to decompose.

While some materials degrade in hours, some take centuries, like plastics. New plastics are being designed specifically to be compostable and degradable. Some of the most promising are PEFs and PETs.

Polyethylene Furanoate PEF – plant based, stronger that PET, requires less mass and better gas barrier characteristics than PET. Less energy to produce (70% less) and 65% less carbon emissions. totally recyclable

Lowering the Carbon Footprint

The first step towards these lower carbon materials will be bio-based. In fact, most of these natural alternatives are not just carbon neutral, but carbon negative! Meaning, that they sequester more carbon by weight than the end product. A perfect example is hemp, which sequesters 1.62 tonnes of carbon for every tonne harvested.

There are rumblings about tradeoffs with food production as many biomaterials in the past that have been used for polymers are crops like sugar, corn and soy. It’s understandable that in a world where many do not get enough to eat, that it might seem incomprehensible that we are growing foods for plastics.

There are recent developments that will certainly ease these concerns:

1. Using bio-waste as a feedstock
2. Growing crops that are not typical food or feed (such as bamboo, flax, jute and hemp).

Biomass or bio-waste products seem to lessen the concern of food vs. plastic as we waste hundreds of millions of tonnes of food each year. In fact some studies say we waste as much as a billion tonnes annually!

That equates to nearly 20% of all domestic waste. So, there is comfort in knowing that those food products are not going to reach anyone’s table, but at the same time, many programs are in place to lessen our wasteful practices with food.

That leaves the better option of growing crops that can be processed into polymers that are not the typical food products. Of these many show promise like flax, jute and hemp. Flax and jute are very fibrous plants and are perfect for textiles without a great deal of processing.

Hemp is also very fibrous, but has the added value of an abundant, “meatier” center section called “hurd”. The hemp plant, when processed properly can yield almost 100% of the harvested material into feedstock for plastics and plastics additives. This, along with its fantastic carbon sequestration rates make it a perfect choice for a bio-based material in plastics.

Other biomaterials that can be processed into polymers include vegetable oils. Unlike corn, sugar and soy, which use the starch from the sugars in the plant, to convert to polymers, these oils contain triglycerides and fatty acids that can be epoxidized into resins. They can also be processed into polyurethanes and used for synthetic rubber.

Bio-based polyesters are some of the more common bioplastics in production today, such as PLA, PBS and PHA. Of these, the most common by far is PLA with current capacity around 250,000 tonnes per year. It is made via polycondensation of lactic acids, and those lactic acids are abundant through fermentation of sugars (raw crops, bio-waste and biomass).

PBS is a more “flexible” bioplastic, more similar to polyfins, with much lower glass temperatures and very high elongation at the break point.

PHAs are showing a lot of promise and could reach scale production soon as there is no need for them to be processed chemically like PLAs and PBSs, but are formed through a reaction with microorganisms as found in substances as common as algae. This means that even the materials used for processing are “carbon-negative”! PHAs also have good O2 and CO2 barrier properties, making them possible bulk packaging replacements for PE and PP.

PEF is a durable (aromatic), bio-based polyester that has the high-performance characteristics of PET. With higher strength and higher glass temperature, it will be useful for more durable, longer-life items. However, PEF requires more rigorous processing and can, under certain conditions, biodegrade much faster than PET.

Bio-sourced polyolefins may someday replace our most common plastic used today (PE and PP). Today PE and PP make up 50% of all plastics and 90% of all packaging, so finding a source to make them more eco-friendly will be a major concern in the coming decades. These bio-based materials have also been referred to as “Drop-in” plastics.

Drop-in plastics, as the name suggests, have the same chemical properties as petroleum based plastics, but are obviously sourced from bio-based materials. They are also similarly easy to recycle.

Innovations in Bio-based Additives

Whether or not a resin comes from petroleum or a bio-based source, all plastics need additives to make them commercially viable. From volume fillers, to plasticizers, to pigments, to fibers added for flexibility and strength, all polymers need additives. For example, PVC flooring is made up of about 20% resin and 80% additives!

The common use of antioxidants, heat stabilizers, and plasticizers during processing is in order to enhance the polymer’s thermal resistance and improve its performance. Light and UV stabilizers are routinely mixed with polymer materials to give them a longer life in outdoor applications. Flame retardants are often sought when there’s a risk for fire hazards. Most of these products are relatively small amounts to alter its intrinsic arrangement, without affecting the material’s specific properties. 

However, a great number of polymer applications involve mechanical strength along with dimensional and morphological stability. In order to meet these requirements, polymer systems are often modified by the addition of reinforcing agents or fillers. These products usually retain their original shape and physical form, producing a separate, dispersed phase inside the polymer matrix. In this way, they alter its pristine structure and can be termed as structural additives.

Additives are mixed with resins to create plastics that have better properties. They can add flame-retardant properties, make the material lighter, add pigment and even improve toughness. Additives can also help to reduce shrinkage during processing; lower warpage during molding, and improve dimensional stability at higher temperatures.

So, while there is a great focus on bio-based polymers (as there should be), real innovation in plastics will need to seriously consider the additive equation also. Bio-based additives offer an alternative to those that are mined or made from petrochemicals, and have all of the positive characteristics listed above and more.

The use of natural minerals in plastics helps them be more durable and reliable. They are also used as a replacement for other additives such as oil to make plastic more flexible. The resins are a type of polymer that is created when polyfunctional monomers are forced to react in a chemical reaction, and are typically synthetic in nature, and play an important role in the manufacturing industry.

They are also used to transform raw plastics into materials that have higher levels of viscosity, glossiness, hardness, adhesion strength, temperature stability and elasticity. Virtually any improvement we make to resins with traditional additives, can be achieved with natural ones.

Additionally, we are finding that bio-based materials can be used as a direct replacement in the form of additives for traditional polymers because they have the same properties as petroleum-based polymers, and of course, they are substantially more environmentally friendly. We have found that by replacing traditional additives with bio-based materials (industrial hemp specifically) we can reduce the plastic’s carbon footprint by up to 44%!

The following are some promising plant-based biomaterials being formally researched as additives. Scientific studies and authors are cited. 

Wood
Wood and its byproducts are interesting fillers for the formulation of biocomposites . They are often used in combination with other components in order to reduce their incompatibility with polymer matrices , facilitate processing , and improve material performance according to specifications for different applications and consumer demands. 

The major component of wood is cellulose. This biopolymer plays an important role in wood – based composites as it confers stiffness to the material. Lignin , the second constituent of wood products, also contributes to the reinforcing action by acting as a binder for cellulose fibrils and allowing the stress transfer from the polymer to the fiber. 

Wood particles may influence the mechanical behavior of composites by acting as nucleating agents toward the matrix. However, in some cases, the wood filler can also produce the embrittlement of the composite with a reduction of strength properties. These topics can be resumed in three basic factors . 

The first is fiber damage during processing and, consequently, a poor wettability of fillers. 

The second is weak bonding between matrix and filler, entailing degradation processes at the interface. 

The third, and most common, is high moisture absorption. Wood – based composites made of virgin materials may have low environmental impact if the wood content is significant .

Bamboo
Bamboo is an important plant fiber with a great potential for composite formulation. The interest toward this biomass as a reinforcing agent stems from its outstanding mechanical properties and the fact that bamboo fibers show specific stiffness and specific strength even comparable to glass fibers. 

Bamboo fibers have been applied in the manufacture of a new type of wood-like composite denoted parallel strand bamboo. Its preparation consists in flattening the bamboo strips into thin strands , impregnating them with phenolic resin, and gluing the impregnated strands under high pressure. The final material shows toughness values higher than wood products commonly used in construction engineering.

Flax
Flax is one of the most frequently used natural fibers in composite materials due to its high specific mechanical properties and moderate cost. Flax fibers are characterized by a high cellulose content and long fiber length .

Recycling of nonwoven PP – flax composites performed by injection and compression molding, and the effects of repeated recycling cycles on the microstructure of the flax fiber and subsequently, the tensile properties of the composites were studied. The results showed that reprocessing produced a decrease in mechanical properties of the material , but the worsening effect was less pronounced for injection molding. In addition, this processing method also promotes good fiber dispersion, high volume fractions, and almost defect-free microstructures that are prerequisites for high strength materials . 

Jute
Jute fiber is one of the most promising natural materials for composite applications . Similarly to flax , it is characterized by a high content of cellulose ( 61 – 72 wt % ) but it also shows a larger lignin concentration ( 12 – 13 wt % ) and a shorter fiber length ( 0.8 – 6 mm ) . Raw jute fibers are organized in cohesive bundles where fiber connection is further improved by the relatively high lignin content of the material . Compared to other vegetable fibers, jute shows better tensile strength than bagasse but lower than flax alone. 

Flax and Jute

Tanguy et al. studied the tensile properties of flax and jute fibers and their relationship with the mechanical performance of the corresponding PP – based composites prepared with 60 wt% filler content . The analysis was carried out on both unidirectional and short fiber composites ; the latter were prepared by extrusion followed by injection molding. 

The experimental results evidenced that flax fiber exhibited higher tensile properties with respect to jute fiber. The corresponding PP – based unidirectional composites showed a good correlation with the above results. However, the reverse relationship was found in the injection-molded samples. In this instance, jute – based composites showed higher Young’s modulus and strength with respect to the flax charged specimen. This difference in behavior was attributed to microstructural effects related to fiber orientation during the injection molding process.

Kenaf

Kenaf fiber is another natural filler with high tensile strength . This feature makes it suitable as a reinforcing agent for biocomposites , especially in the production of automotive components . 

From the MDPI Polymers Journal, Chee, et al. investigated the addition of montmorillonite clay to a kenaf – based PLA composite . Samples were prepared by using 30 wt% organic fiber and 0 – 3 wt % montmorillonite. An improvement of flexural and tensile strength was found for kenaf – PLA biocomposites. The addition of the inorganic charge produced a further enhancement in mechanical properties. This outcome was recorded at low content of cofiller and was attributed to the good matrix–fiber interaction , which improved load transfer capacity 

Bamboo and Kenaf

Some have used bamboo mat to improve the properties of woven kenaf-reinforced epoxy composites. Woven kenaf/bamboo hybrid composites were fabricated by hand with different ratios of kenaf and bamboo fibers. With respect to pure kenaf composites, hybrid formulations showed a reduction in water absorption and thickness swelling along with an improvement in flexural properties. It was found that the composite with a 50/50 kenaf/bamboo ratio exhibited the best overall performance. This composition also showed the higher impact strength, even beyond the values of pure kenaf and bamboo composites.

Hemp

Hemp plant belongs to the Cannabis species but, differently from Cannabis indica L. It contains only low quantities of the psychoactive compound tetrahydrocannabinol (THC). To qualify for “industrial hemp” the samples must test below 0.3% THC – effectively non-existent.

After jute, hemp is the largest grown bast fiber in the world, and also recognized as the strongest. The growing interest toward this natural fiber as a reinforcing agent in polymer composites is relatively new because of the excessively tight regulations on cannabis in the United States. Some would say industrial hemp was unfairly lumped into the same restrictive category, but as of the Farm Bill of 2018, it is now legal to grow and transport freely. 

Historically, hemp has been used in a great number of applications such as the production of paper, textiles, automotive components, and building materials. 

The effect of hemp fiber addition in polymeric matrices has been studied recently. Dayo et al. found that polybenzoxazine composites charged with 30 vol% filler showed improved performance in flexural and impact strength. In addition, they demonstrated that by reducing fiber dimensions, a further increase in mechanical properties was achieved. 

A similar effect of fiber content and fiber dimensions was observed for water uptake. All the studied composites exhibited an enhancement in water absorption with the increase in vol % of fiber and the decrease in the fiber dimensions. In a previous study, the same authors investigated the effect of alkali-treated hemp fiber in curing behavior, and mechanical and thermal properties of polybenzoxazine composites. 

Samples were prepared by sandwiching a fiber film between two benzoxazine prepolymer layers. Next, the obtained compound was submitted to hot press curing. The dynamical mechanical characterization of the cured specimens showed that by increasing the vol % content of fiber, a regular enhancement in storage modulus and an increase in glass transition temperature were recorded. Hemp fiber also positively affected the flexural properties of the composites. 

In this instance, both flexural modulus and strength increased with fiber loading reaching a maximum of 20 vol% . The reported effects were attributed to the ductile nature of natural fibers, the defibrillation of the fiber during the alkali treatment, and the strong hydrogen bonding effect between continuous phase and charge. This interaction improves the adhesion between the composite components and helps the transfer of the stress from matrix to the reinforcing agent.

Hemp is gaining popularity in both the agricultural and scientific arenas. It is easily grown and processed. It can then be transformed into a variety of raw material types that serve a myriad of industries. The possibilities for plastics and polymers seems endless, considering the material has only been available for wide-spread adoption in the scientific community for just a few years. Many polymer researchers we speak to are most excited about the possibilities for industrial hemp. 

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