Climate Disrupted: Lignin, Cellulose, and Starch Bioplastics, TSPs

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In our series on 3D printing in a climate-disrupted world, we have been exploring the variety of biodegradable bioplastics that could be used to replace petro-based materials. We have so far looked at polylactic acid (PLA) and its more biodegradable companion polyhydroxyalkanoate (PHA). Here we look at a few other biodegradable bioplastics, starting with thermoplastic starch polymers (TSPs).

Thermoplastic Starch Polymers

Not to be confused with PLA, which is made from the lactic acid derived from starches, TSPs are made up of the starches themselves. They are hydrophilic, difficult to process, and very brittle at room temperature. They have to be blended with plasticizers in order to overcome these properties. Though starches are biodegradable, the plasticizers with which they are blended may not be.

When blended with the following plasticizers, TSPs are biodegradable: PLA; polycaprolactone (PCL), a biodegradable polyester usually derived from benzene (a constituent of crude oil); and polybutylene-adipate-co-terephthalate (PBAT), a biodegradable random copolymer derived from petroleum that is often used in compostable cling wrap and plastic bags; polyvinyl alcohol (PVA), a petro-derived water-soluble synthetic polymer; and polyethylene oxide (PEO), a polyether compound derived industrially from petroleum.

Most TSPs have been limited to packaging applications, due to the properties discussed above, but there are interesting research endeavors in the works that blend TSPs with other materials, such as unripe coconut fibers, in order to improve their mechanical strength and longevity without sacrificing overall biodegradability.

Cellulose

As the primary component of cell walls in plants, cellulose makes up about one-third of the vegetable matter on the planet and is the most abundant polymer on Earth. Despite its prevalence, commercial cellulose comes from just two sources, the cotton and wood industries. While cellulose is typically used for the production of paper products, a small percentage is extensively modified to create cellophane, rayon, cellulose acetate, and cellulose ethers.

Invented in 1856, celluloid is considered the first thermoplastic and was used as a replacement for ivory and for filmstock. Though it was eventually replaced by the less flammable and less expensive cellulose acetate, celluloid is still used today for ping-pong balls, guitar picks and other musical instruments. Cellulose acetate continues to be a base for film, as well as a material used for coatings, eyeglass frames, playing cards and cigarette filters.

a) Schematic summary of the protocol for the 3D printing of alginate and CNFs mixtures and subsequent aerogel fabrication. Photos of b) 3D‐printed aerogel grids before (left) and after (right) drying, c) 3D‐printed aerogels of cylindrical shape, and d) layered aerogels that were 3D‐printed by a dual‐extrusion approach where the dark layers contain a mixture of CNFs, alginate, and PEDOT:PSS, and the translucent layer a mixture of CNFs and alginate (the samples shown on (c) and (d) have been soaked in water). Image courtesy of Advanced Functional Materials.

There are a number of research endeavors related to 3D printing with cellulose materials. In the past couple of years alone, there have been studies into the ability to 3D print cellulose aerogels, biodegradable wound patches, filament made from recycled cellulose polypropylene, cellulose acetate, responsive ink, and large-scale objects. Biomaterials company UPM has developed a form of what it claims is a cellulose-PLA filament that is already on the market. We have reached out to the company to confirm that this material is distinct from the sawdust-based wood filaments that have long been on the market.

Lignin

Lignin is the second most abundant prevalent biopolymer after cellulose, making up about 30 percent of a plant’s structure and serving as the glue that holds cellulose fibers together. In industrial sectors, it is a byproduct of the production of paper, ethanol, plant-derived chemicals, and pharmaceuticals, with 50 million tons of lignin created by the chemical pulp industries annually. When heavily modified, lignin can form the basis of thermoplastics with the physical characteristics of polystyrene, lending itself as potentially useful for biodegradable cups, bags, and packaging. However, most lignin is burned.

As of 2020, 22 companies are involved in commercially producing lignin in amounts above 5,000 tons annually and lignin-based plastic is not yet manufactured at industrial scales due to the fact that very few organisms can break the material down. In turn, researchers are developing strains of bacteria that can feed on the material and produce the necessary plastic precursors in the process.

Photographs of: PLA and PLA coated pellets (A); lignin and tetracycline containing PLA filaments (B); lignin and tetracycline containing 1 cm × 1 cm squares prepared using 3D printing (C); and different shapes printed using the filament containing 2% (w/w) LIG (D). Image courtesy of Pharmaceutics.

As with cellulose, there are currently numerous research efforts underway to 3D print with lignin, often blending the material with PLA, but also combining it with nylon. And, due to its biodegradability and biocompatibility, lignin has been explored as a material for 3D-printed, implantable medical devices. Previously, there was one lignin-PLA filament on the market from Two Bears, but given the difficulty of scaling lignin plastic production, it may be possible that this material was papermill pulp blended with PLA.

Oak Ridge National Laboratory performed research into 3D printing a nylon composite made up of 40 to 50 percent lignin by weight with 4 to 16 percent carbon fiber. Image courtesy of Oak Ridge National Laboratory.

In the next post in our series, we’ll look at two more intriguing materials that could serve as the basis for new bioplastics and that you’ve surely eaten, either intentionally or incidentally: chitin and keratin.

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