Keywords

1 Introduction

Alongside metals and minerals, plastics are the most commonly used materials in additive manufacturing. The main advantages are their ease of processing, moderate material costs and low weight [1]. However, the areas of application for additively manufactured components made of plastics have so far been limited due to their comparatively low strength. For this reason, processes that offer additional reinforcement of the plastic matrix with short and long fibers made of high-strength materials (e.g. glass or carbon fibers) have been used for some time. Very high strengths can be achieved in particular by inserting long fibers made of carbon [2]. This makes it possible to additively manufacture highly resilient and at the same time lightweight components that are used, for example, in vehicle and aircraft construction. However, these components represent a composite of different materials produced on the basis of fossil raw materials, which is difficult to recycle and generally not biodegradable.

Therefore, the question arises as to how these conventional materials can be replaced by bio-based and biodegradable materials? In particular, it must be clarified which material properties the biomaterials can offer? In addition, it is of great interest which economic and ecological consequences the use of biomaterials has?

In this contribution, the potential of a new composite material is investigated, the matrix of which consists of a bio-based plastic. In this investigation, it is assumed that the matrix is reinforced with a fiber material made of natural fibers to significantly increase the strength. Both short and particularly long fibers can be used. The goal is to determine the technical, economic, and environmental potential for developing a bio-based composite material for additive manufacturing. This potential material should have a lightweight yet strong structure and be biodegradable after use under controlled conditions. In addition to the economic potential, the environmental impacts that may arise with the industrial use of bio-based materials will be investigated. In particular, land grabbing in subtropical and tropical wetlands and the associated reduction in biodiversity as well as competition with food cultivation are to be contrasted with the saving of fossil resources and the reduction of the greenhouse effect.

2 Investigation of Potential Composite Materials

Composites based on plastics and fibers open up great potentials, since the properties of the individual components can be surpassed in the composite of a plastic and a fiber. The fibers represent the load-bearing component embedded in the plastic [3]. The matrix material plays an important role in the composite by holding the fibers in place, transferring stresses between the fibers, protecting the fibers from adverse environmental conditions, preventing surface abrasion, and supporting the fibers under compressive loads [4]. The combination of Additive Manufacturing and fiber reinforcement opens up a new field of fiber-reinforced Additive Manufacturing (FRAM). The materials used in this process are shown in Fig. 1.

2.1 Matrix Materials

When selecting the matrix material, attention should be paid to compatibility with the reinforcing fibers. For thermal compatibility between matrix and fibers, it is important that the thermal expansion coefficients are relatively similar so that cracks or delamination between matrix and fibers do not occur during temperature fluctuations. A good bond between fiber and matrix is the basis for physical compatibility. Similarly, mechanical stress should not cause the two composite partners to separate. If the two materials do not react chemically, there is also chemical compatibility over a long period of time. This ensures that both materials retain their desired properties over the entire life of the component [4]. On the one hand, composites offer many possible combinations of different plastic and fiber materials, and on the other hand, material properties such as strength and stiffness can be specifically adjusted by the quantity, position, length and orientation of reinforcements fibers. The highest reinforcement is achieved when the fibers are continuous and run in the direction of the load (long fibers). Therefore, material properties can vary not only between components but also within a component [3].

Fig. 1.
figure 1

Overview of possible matrix and reinforcement materials in fiber-reinforced additive manufacturing (FRAM) (based on [4])

Full size image

2.2 Potential Reinforcement Materials

Basically, fibers used for reinforcement should be thin and flexible to allow easy insertion into the matrix material. In addition, they must also have high strength and elasticity in order to be able to compensate for the shortcomings of the matrix material. For many years, glass fibers and carbon fibers have been used for this purpose because they offer extremely high strength. In addition, natural fibers (NF) have also been incorporated for some years. These include both animal-based natural fibers (e.g. wool or silk) and plant-based fibers (e.g. kenaf, sisal, etc.), which are the focus of this contribution. Both types of NF offer some advantages over conventional fibers, such as low cost and good recyclability [5]. Natural fiber can also offer some advantages in processing, such as significantly reduced abrasion of nozzles in additive manufacturing. Possible disadvantages of natural fibers include limited strength and low melting point, as well as possible moisture absorption, which can limit processability [6]. To overcome the disadvantages, some treatment methods for natural fibers have already been developed. These include, for example, bleaching of the fibers to improve the mechanical properties [6, 7].

3 Additive Manufacturing Using Reinforcing Fibers (FRAM)

A high degree of flexibility in production is provided by Additive Manufacturing (AM). These processes are characterized by the fact that components are manufactured generatively in layers and directly without tools. This enables the individual design of components with a wide range of geometric complexity [4]. The fused filament method (FLM) has proven to be a particularly simple and robust process, which is also significantly more cost-effective than laser-based processes. In AM, conventional materials such as ABS or nylon are primarily used. In addition, bio-based materials such as PLA are also used, which can also be used as a matrix material in a composite material. The processing of continuous fibers within the FLM process is a new approach in AM, which has so far been used commercially primarily for the incorporation of conventional fibers. Different technologies are used, which differ mainly in how the strands of continuous fibers are impregnated and how these fibers are processed with the matrix materials [8]. Currently, the most common way to process short fibers with the FLM process is to incorporate the fibers into the filament of the matrix material. All axes of the 3D printer are driven by stepper motors. While the X and Y axes and thus also the print head move according to the paths calculated by the slicer software, the filament with the matrix material is feed in the nozzle, melted and deposited on the building platform. This builds up the component layer by layer. If a long fiber is to be processed in addition to the filament, another motor is used to convey the fiber. First a layer of plastic is extruded and then the fiber is laid over it in the desired orientation. [9].

4 Economic and Environmental Considerations

The total global market for composite plastics in 2021 was 12.1 million tons, almost back to the level of 2018 [10]. The share of thermoplastic composites is over 50%. The European market has a share of approx. 25%. The share of biodegradable bio-based plastics in the overall plastics market is still below 1% [12]. Production capacities for PLA are estimated to have tripled since 2016 [4]. At the same time, a market survey forecasts an 8-fold growth in ten years to over 9 billion USD in 2028 for the additive manufacturing market with fiber-reinforced plastics [13]. Both developments, as well as the existing use of bio-based PLA as a filament and of conventional glass and carbon fibers for AM with conventional plastics, indicate an enormously fast-growing market for bio-based and plant-fiber-reinforced plastics.

4.1 Matrix Materials

With an increasing market share, the production and life cycle of biodegradable bio-based plastics have been investigated for some time with regard to their environmental impact. PLA, which is already being additively processed, dominates as a plastic. The production routes from maize and sugar cane have been evaluated by Life Cycle Assessments (LCA) with regard to various environmental categories [14, 15]. The production of PLA from sugar cane with 2,334 gCO2/kg causes approx. 27% less GHG emissions than PET from fossil raw materials, which can also be additively processed, with 3,200 gCO2/kg. If the bound CO2 of 1,800 g/kg PLA is considered, the GHG emissions are approx. 84% lower. This contrasts with the significantly higher land use of 1,775 m2 per ton of PLA. This land use can be considered for on the basis of the annual storage capacity of forests of 10 t CO2/ha by a missing annual CO2 storage of 1,775 gCO2/kg PLA (see Fig. 2). For the disposal of biodegradable plastics, studies show that, similar to conventional plastics, mechanical recycling and incineration are preferable to composting or landfilling [16, 17]. Without accounting for land use, GHG emissions are lower for PLA than for PET in all three EOL scenarios: landfilling, power from WIP (Waste Incineration Plant) and heavy oil substitution in high temperature processes. When land use is considered, Fig. 2 shows that apart from mechanical recycling (not shown), only the addition of heavy oil substitutes in high-temperature processes is more climate-friendly than the landfilling of PET.

This is in contrast to the extremely low degradation rate of PET and the accumulation of microplastics in the environment during wild dumping. Considered for one year of product life cycle, the use of PLA instead of PET is always advantageous, except for the EOL scenario landfilling. Extrapolated to the annual global plastic consumption in glass fiber reinforced plastics of approx. 5 million tons, approx. 8,875 km2 of arable land would be required in warm, humid regions. In return, 13.5 million tons of CO2 could be saved, which corresponds to the annual binding by approx. 13,500 km2 of forest and significantly overcompensates for land use.

Fig. 2.
figure 2

Comparison of CO2 emissions of PET and PLA without and with accounting for land use as missing CO2 storage for 1 year.

Full size image

4.2 Fiber Materials

Similar to bio-based plastics, studies on life cycle assessments have primarily shown savings in fossil energy and greenhouse gas emissions and comparatively higher land use [18,19,20]. The production of natural fiber using the example of kenaf consumes comparatively less than 10% of fossil energy and generates about 30% of greenhouse gas emissions without considering CO2 storage [18]. This corresponds to the EOL scenario of landfilling, where the stored CO2 is released (see Fig. 3). The land use of 1,040 m2 of arable land per ton of kenaf can again be converted into a missing annual storage capacity of 10 t/ha of forest [19]. As a result, the EOL scenario Landfill is slightly advantageous for fiber, while Power from WIP is only possible for kenaf and shows clear advantages. Again, extrapolated to the annual global glass fiber consumption in plastics of about 5 million tons, this would require about 5,200 km2 of farmland. In return, 7.3 million tons of CO2 would be saved in the EOL scenario Power from WIP, which corresponds to the annual binding by approx. 5,400 km2 of forest.

Fig. 3.
figure 3

Comparison of CO2 emissions of glass fiber and kenaf fiber without and with accounting for land use as a missing CO2 reservoir for 1 year.

Full size image

4.3 Fiber-Reinforced Polymer Materials

The comparison of polymers and fiber materials with regard to greenhouse gas emissions without and with consideration of land use in the form of missing CO2 storage capacity now leads to a comparison of glass fiber reinforced PET (PET GF) with PLA reinforced with kenaf fiber (PLA KF), which has 70–90% of the tensile strengths. Both composites are considered with a realistic 50% weight share of fiber material. Without taking land use into account, PLA KF causes about 60% of the greenhouse gas emissions of PET GF with the EOL scenario Landfill and about 30% with the EOL scenario Power from WIP (see Fig. 4). If land use in the form of lost forest storage capacity is included, PLA KF is significantly above PET GF with the EOL scenario Landfill, while the EOL scenario Power from WIP leads to 72% of the GHG emissions. The most climate-friendly EOL scenario here is also mechanical recycling, whereby fiber shortening during shredding leads to lower strength. The contribution of the use of natural fiber-reinforced degradable biopolymers to climate protection with the inclusion of land use is mainly determined by the EOL scenarios for a product lifetime of one year. If the product lifetime doubles, the influence of land use on the CO2 balance is halved. Composting or landfilling lead to the release of the stored CO2, whereas with PET GF this lack of effect leads to a better balance. Nevertheless, the biodegradability of composites made of biopolymer and natural fibers reduces the environmental impact of wild dumping. Microplastic emissions are estimated at 5–18 million tons of plastic worldwide [21]. The share of thermoplastic emissions for Germany is estimated at 38% in the same study. For conventional plastics, a degradation time of 2,000 years is assumed, which leads to the already visible and problematic accumulation in the environment.

Fig. 4.
figure 4

Comparison of the CO2 emissions of PET GF and PLA KF without and with the inclusion of land use as a missing CO2 reservoir for 1 year.

Full size image

5 Conclusion

The present analysis shows that the use of composites based on bio-based and biodegradable materials is possible. Suitable matrix materials as well as reinforcing materials in the form of natural fibers are available for this purpose. The feasibility of these bio composites has already been extensively demonstrated. Additive manufacturing in the form of FFM is particularly suitable for implementation, as the insertion of fibers can be excellently integrated here. The economic and ecological analysis shows that the use of biomaterials makes a significant contribution to climate protection compared to conventional materials, if the products are used for a long time and recycled for materials or energy after use. However, these potentials are not yet being exploited on a large scale, as natural fibers are hardly available for additive manufacturing. In contrast to conventional composites, there is also a lack of suitable manufacturing systems that allow simple and safe processing of natural fibers in conjunction with bio-based matrix materials.