High-quality plastics are indispensable in many industries, such as aerospace, mobility, and medical technology. They offer compelling advantages like low weight, excellent formability, and durability, as well as cost-effective production methods. At the same time, the fossil fuels required for their production are finite, and the disposal of plastic items is challenging due to their persistence. For these reasons, environmental compatibility and sustainability are being driven forward in the plastics sector to harness these benefits in a sustainable way. In this blog post, we will highlight the sustainable plastic materials that are available—from bioplastics to recyclates—and discuss the opportunities and challenges associated with their use.
“Sustainable plastics” is a very broad field. It encompasses various categories of materials, such as the large group of recyclable fossil-based (petroleum-based) plastics. A distinction is made here between whether they are mechanically (as regrind or regranulate) or chemically recycled, as this impacts the subsequent properties of the recycled product. Also of fossil origin, but still considered sustainable, are plastics that are manufactured using renewable energy.
The most environmentally friendly category of all sustainable plastics is the so-called bioplastics, which are newly produced (predominantly) from renewable resources. These are further subdivided:
Plant-based plastics are derived from the starch or cellulose of plants such as corn or sugarcane. Bioplastics created by microorganisms, such as polylactic acid (PLA) or certain polyhydroxyalkanoates (PHA), are also biodegradable and made from renewable resources.
Deviating Properties of Bioplastics
As part of a test series, RKT tested and evaluated several biopolymers. These included the biodegradable plastics PHB, PLA, and blends thereof.
For the most part, the properties of biodegradable plastics differ from those of fossil-based plastics, particularly regarding their mechanical characteristics, such as strength and stability. Both aforementioned types of plastic are not nearly as rigid as conventional plastics; instead, they are considerably more ductile and elastic. They are therefore well-suited for applications like single-use products (cutlery, plates, etc.) or short-lived items such as packaging. Their strength is also sufficient for simple housings.
The thermal properties of the bio-variants are also different. They have significantly lower melting temperatures and are therefore not suitable for applications requiring thermal stability. Polylactic acid, for instance, begins to soften at just 55 to 65 degrees Celsius. Moreover, petrochemical plastics cover a much broader spectrum of properties, and they also have the edge when it comes to requirements like transparency and conductivity.
It is possible to blend the aforementioned bioplastics to achieve slightly modified properties. Fundamentally, their characteristics are quite similar. Additives, like those used in fossil-based plastics, can also be incorporated to make the bioplastics harder and more stable, for example, with glass fibers, carbon fibers, or glass beads. However, this affects the flow properties, so it must be tested to determine how well the bioplastics can still be processed with these additions.
What makes these two groups of bioplastics particularly environmentally friendly is their biodegradability, which, however, depends on specific conditions. Simply throwing these plastic products on a home compost heap and hoping for quick decomposition is shortsighted. Despite their plant-based origin, the degradation process would be very slow. Ideal conditions for the degradation of these plastics are found in industrial composting facilities, where heat and microorganisms significantly accelerate the decomposition process.
Bioplastics That Are Not Biodegradable
A relatively small branch within the bioplastics sector consists of plastics made from renewable resources, such as castor oil, that are not biodegradable. A polyamide can be produced from castor oil that possesses nearly identical mechanical properties to its petrochemical counterpart.
As part of its test series, RKT also tested a polyamide derived from renewable castor oil.
Shopping cart chips made from renewable castor oil polyamide
Currently, however, this type of plastic is significantly more expensive because the extraction, refining, and processing of these oils are complex and costly.
Sustainable Plastics from Recycling
The term ‘sustainable plastics’ primarily refers to recycled fossil-based plastics that are reprocessed for reuse. A distinction is made here between physical and chemical recycling.
For the physical recycling process, the plastics must first be sorted by type (e.g., PET bottles) before being ground down, possibly remelted, and turned into regranulate. The more frequently plastics are ground and remelted, the more they degrade, as the length of the molecular chains shortens with each cycle. As a result, the recyclates gradually lose some of their mechanical properties. It is possible to blend in virgin material to preserve these properties. For certain products that require large amounts of material but have less stringent demands on high mechanical stability, such as housings or vehicle wheel wells, recycled plastics like polypropylene recyclate are a good alternative.
Chemically recycled fossil-based plastics largely retain their original mechanical properties. Plastic waste—including mixed types and contaminated materials—is broken down into its source materials, or monomers, through chemical processes. Possible methods include processes like depolymerization or pyrolysis. This creates fossil-based raw materials that exhibit nearly the same high quality as virgin material, making them suitable for more critical applications, such as in the medical technology or food industries. Chemical recycling, therefore, offers great potential for reprocessing plastics that are not suitable for mechanical recycling.
Historically, criticisms of chemical recycling have included the high energy consumption of the processes, the creation of too many toxic by-products, and a low output of new material. In recent years, however, these processes have been optimized to the point that energy consumption has dropped significantly and is not substantially higher than that of physical recycling (regranulation), resulting in much larger quantities of reprocessed plastic material. This makes the recycling method more economically attractive and allows it to be seen as a complement to conventional physical recycling.
Influence of Bio-based and Recycled Plastics on Tooling and Processes
Recycled Plastics:
While the impact on the molds themselves is minor and does not differ from that of fossil-based plastics, the injection molding process must be adapted for recycled materials. For the most stable process, it is best when the ground materials from mechanical recycling are first regranulated. Although it is possible to use the regrind directly in the injection molding machine, its particle size varies from very fine to coarse. This inevitably leads to process fluctuations, which is entirely undesirable, especially for medical products that require an absolutely stable process.
When the recycled material is converted back into pellet form after regranulation, a more stable process is achievable. It should also be noted that the flow behavior of the molten regranulates changes slightly due to the shorter molecular chains; the material becomes somewhat more free-flowing. Consequently, flowability and injection pressures will change.
With chemically recycled plastics, there is almost no impact on tooling and processes. This is because the plastic waste is completely broken down into its constituent parts and reprocessed, resulting in a material that is largely equivalent to virgin material.
Bioplastics:
The aforementioned bioplastics from renewable resources, such as cellulose, fatty acids, or polylactic acid, generally allow for a stable injection molding process. However, the processing parameters will differ from those of fossil-based plastics: The melting temperatures are lower—for example, PLA melts at around 150 degrees Celsius—and the melt is typically more viscous.
This affects, among other things, the gating system during the filling of the part. Due to this higher viscosity, it may be necessary to increase the cross-sectional area of the gates leading to the plastic article. However, this is not a characteristic unique to bioplastics. As a standard procedure, the material data sheet for any plastic pellet is used to determine the necessary mold and process design, including factors like evaluating the required draft angles or calculating shrinkage.
Conclusion and Outlook
The push for sustainability in the plastics sector is advancing, and we at RKT are actively helping our customers navigate this transformation. While the circular economy concept is being driven by the reuse of plastics and the optimization of recycling processes, bioplastics from renewable resources are also establishing themselves as a forward-thinking alternative for specific applications.
Companies looking to improve their environmental footprint often face the question: Recyclate or bioplastic? The answer depends on the specific application. At RKT, we analyze your specific requirement profile and recommend the most suitable material solution. For example, by strategically using a bio-based polyamide (PA) for a technically complex housing component, we were able to significantly reduce the part’s carbon footprint without compromising on mechanical stability. At the same time, we have extensive experience in processing high-quality regranulates, which are ideal for components where circularity is a top priority.
Due to more complex manufacturing and reprocessing methods, using bioplastics and quality-assured regranulates is often associated with higher initial costs compared to conventional fossil-based plastics. However, we see this as an investment in the future that pays off in the long run—not just for the environment, but also for your company as it positions itself as an innovative leader in the market.
Your path to a sustainable product with RKT
If you are interested in bioplastics or regranulates and are already planning a project, please contact us; we will assist you in selecting materials and planning processes and take care of the entire production of your plastic parts made from bioplastics.
