Natural substance 3-carene is a part of turpentine oil, a byproduct of the cellulose production process from wood. So far, this byproduct has mostly been incinerated. Researchers at Fraunhofer are using new catalytic processes to convert 3-carene into building blocks for bioplastics. The new polyamides are not only transparent but also have high thermal stability.

Plastics are a valuable alternative to glass or metal for a wide range of applications. Polyamides play an important role in producing high-quality structural components as they are not only impact and abrasion resistant but also resistant to many chemicals and solvents. Nowadays, polyamides are mainly produced from crude oil.

A Sustainable and Renewable Alternative

The Fraunhofer IGB Institute for Engineering and Biotechnology is investigating a sustainable and renewable alternative for producing new plastics with high efficiency from terpenes (a group of organic compounds abundant in nature, mostly found as the main components of essential oils in plants. Many terpenes are hydrocarbons, but there are also compounds that are not hydrocarbons and contain oxygen in their molecular structure, yet they are considered terpenes, such as alcohols, aldehydes, or ketones. Terpene hydrocarbons have a molecular formula of C5H8)n). Terpenes are rich in resin in wood. Natural materials from coniferous trees such as pine, European spruce, or fir are available. In the production of pulp, where wood is shredded to separate cellulose fibers, terpenes are separated in significant amounts as a byproduct, namely turpentine oil.

Monomers from Wood Waste

In the joint project "TerPa, Terpenes as Structural Components for Bio-based Polyamides," researchers at the BioCat Straubing unit of Fraunhofer IGB have successfully optimized the synthesis of lactams from terpene 3-carene and converted them into a competitive, potentially scalable industrial process. Lactams are building blocks for producing polyamides. Straubing experts have shown that terpenes such as alpha-pinene, limonene, and 3-carene are suitable raw materials for synthesizing bio-based lactams.

Cost-Effective Single-Step Reaction Sequence

The conversion of 3-carene into the corresponding lactam requires four consecutive chemical steps. The special feature of the Straubing patent-pending method is that the conversions can be carried out as a "cost-effective single-step reaction sequence" in a separate reactor - meaning there is no need for the purification of intermediate products. Paul Stockmann, who has developed and optimized this promising process, explains: "By carefully selecting catalysts and reaction conditions, we have achieved this important milestone, making the method time and cost-effective."

Stockmann said: "Even on a laboratory scale, our process delivers more than 100 grams of pure lactam monomer in each production cycle. This amount is completely sufficient for initial research, production, and evaluation of new plastics." Another advantage: no toxic or environmentally hazardous chemicals are required for lactam synthesis.

Bio-based, Transparent, High Thermal Stability

However, that's not all. Due to the unique chemical structure of 3-carene, the natural compound side chains prevent the crystallization of the resulting polymer. Dr. Harald Strittmatter, head of this project at the BioCat unit in Straubing, says: "As a result, our bio-based polymers are mainly 'amorphous' and therefore transparent, which is very unusual for bio-based polyamides." This makes the new polyamides suitable as protective shields, for example in helmets or ski goggles. They can also be produced with much lower energy input compared to petroleum-based transparent polyamides. Unlike other bio-plastics mainly produced from starches of corn, wheat, or potatoes, bio-based polyamides do not compete with food production. Instead, they add value to waste streams that have been incinerated for energy production.

Another advantage: the new bio-based polyamides have excellent thermal properties. Strittmatter says: "The glass transition temperature of our polyamides is 110 degrees Celsius. Therefore, they can always withstand high temperatures, for example as components in vehicle engine compartments." It is true that polyamides made from fossil sources have similar thermal properties. However, due to their aromatic domains absent in polyamides based on 3-carene, they change color over time under UV light, limiting their suitability for outdoor applications.

Carenelactams Provide New Properties to PA12 and PA6

Scientists have also polymerized bio-based lactams with other commercially available monomer molecules, namely laurolactam (monomer PA12) and caprolactam (monomer PA6), to form copolymers. The crystallinity and hence the transparency of the new copolymers have been significantly modified. In essence, the application profiles of the commonly used plastics PA12 and PA6 have potentially expanded.

Following further optimization of monomer synthesis, colleagues at the Fraunhofer Institute for Environmental, Safety, and Energy Technology UMSICHT in Oberhausen will scale up this process to a 20-liter pilot scale and produce larger quantities of lactams. The properties of the new polymers and copolymers will then be further examined in more detail to identify potential applications. Researchers also intend to study the biodegradability of the new polyamides. Fraunhofer researchers hope that interested companies can eventually transfer the results to an industrial scale.

In crystalline polymers, polymer chains align regularly in one direction. Incident light is scattered on the scattered crystalline structures, making plastics appear matte or opaque. On the other hand, if polymer chains are irregular (due to interference of side chains with each other), we talk about amorphous polymers. Incident light is not scattered; polymers appear transparent.

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