Development of Interior Trim Coverings Made from Composite Material with Natural Fiber-Reinforced Biopolymer Matrix

Polymeric materials, which began entering our lives approximately 100 years ago, continue to transform how we live by replacing many traditional materials as technology advances. Polymers, used in numerous sectors due to their superior mechanical properties, easy formability and low density, have become an essential material for the automotive industry as well. When the performance of polymers proved insufficient, polymeric composites began to be used. Initially, such composite materials were developed using thermoset polymer matrices. Despite achieving very high mechanical values and being extremely lightweight, the major drawback was the lengthy process times unsuitable for production processes like automotive manufacturing where seconds matter. Furthermore, the non-recyclability of these materials and volatile organic compounds (VOCs) that could be released during production led to thermoplastic composites taking their place. With the development of thermoplastic composites used as compounds in injection molding, prepregs or nonwoven surfaces began to be thermoformed. This made it possible to produce parts that were recyclable, had considerably high mechanical strength and were equally lightweight. But would this development be sufficient for the automotive industry? Increasing population and urbanization worldwide and in our country, along with changing living conditions and technology, have raised expectations for vehicle comfort and safety alongside vehicle numbers. This increase and the additional equipment added to vehicles has caused present-day vehicle weights to nearly double compared to 50 years ago. Parallel to this increase, internal combustion engine technologies and alternative fuel engine technologies that could not keep pace with development have failed to prevent vehicles from rapidly polluting our world. For this reason, automotive companies have turned to lightweighting their vehicles and have begun working rapidly to meet emission targets accepted through international agreements. CAFE regulations lead these legal provisions. This regulation aims to progressively reduce CO² emissions from passenger vehicles to 95 g/km. It should be noted that this value is given per manufacturer, not per vehicle. For this reason, companies have begun rapidly introducing electric vehicles alongside internal combustion engine vehicles to the market. In electric vehicles, lightweighting is also among the most important requirements to increase the distance that can be traveled on a single charge.

CAFE Regulations

Automobile weights that have reached a certain level with thermoplastic composites have not yet been reduced to the desired level. For this reason, different reinforcement materials began to be used. Natural fibers lead among these materials. In addition to compounds with natural fiber additives, woodstock materials (50% polymer + 50% wood chips) that can be thermoformed have begun to be used extensively.

Woodstock Sheets

Another structure currently used is the combination of polymer and natural fiber in nonwoven form, that is, as felt, and shaping through thermoforming. Such materials can be produced in the desired part thickness and weight. Within the scope of our project, the target was not only weight reduction but also reducing the carbon footprint of the part to the lowest level. For this purpose, a part currently produced through injection molding from PP (polypropylene) with 20% talc additive was used as the baseline. The goal was to produce this part by replacing PP with a bio-polymer, talc with natural fiber, and injection molding with thermoforming. As is known, 6 tons of CO² emissions are generated during the production of 1 ton of PP. Through this work, in addition to weight reduction, we aim to reduce the CO² emission values that will occur during PP production. As a result of conducted research, PLA (polylactic acid) was found to be the most suitable polymer for the part in terms of mechanical, thermal properties and manufacturability. Polylactic acid (PLA) biopolymer plays an important role in companies realizing their sustainability policies since it is produced using renewable sources. It is mainly produced through the polymerization of lactic acid obtained through the fermentation of sugars derived from plant sources. Among bioplastic varieties, PLA is the polymer with the closest price characteristics to conventional plastic raw materials, which is why it has found widespread applications in recent years. However, particularly due to its brittleness and low impact resistance, and because processing it in conventional plastic processing machines is more difficult, it is mostly used as the main component of various compounds. Due to its biodegradable properties, many food packages are also produced from this material. In the project, jute fabric or UD fibers were placed between two PLA layers obtained in sheet form with desired properties and dimensions to create a sandwich structure, which was then laminated together using heat and pressure. This work was carried out under the supervision of Assoc. Prof. Güralp Özkoç, faculty member at Kocaeli University.

Sandwich structure

PLA + Jute Layer

Sample bars were taken from the produced sheets and mechanical values were measured. In this work, to enable the automotive interior trim part targeted for production to be mounted on the vehicle, the PLA thickness on one surface of the layer was increased to allow welding of the required connection posts onto the part.

PLA + Jute Layer Cross-Section View

Tensile, DMA (Dynamic Mechanical Analysis) and moisture absorption tests were performed on PLA + jute composite sheets prepared in different ratios. In the evaluations conducted, the mechanical values of the produced material were found to be quite suitable according to automotive specifications. However, due to the layered structure of the material, difficulties were encountered with detail formation. Additionally, due to the production technique being quite difficult and having a line orientation issue, it was concluded that it could not be used in the specific part desired for automotive applications. At this stage, work began on producing sheets similar to PP + hemp felt structures that were being produced under a different project at the time. Although there was initial concern that difficulties would be encountered due to PLA's mechanical properties, the obtained PLA fibers proved to be quite suitable for the process.

Hemp Fibers PLA Fibers

PLA + hemp fibers were processed in the felt production process and manufactured at 1600 g/m². These felt structures were heated in contact and infrared ovens at specific times and temperatures and shaped under pressure.

Contact Oven

Heated PLA + Hemp Felt on Form Mold

Shaped Product

In shaping such materials, it is suitable to use hot material and cold molds. Through a frame placed inside the mold, the produced part can be covered with an appropriate fabric. A polyester non-woven fabric was used in this process. The connection points that will enable the part to be mounted on the vehicle will be added through design modifications to the part. Since our project is also being continued as a TEYDEB project with main industry partners, we are unable to provide information at this stage regarding the work carried out in felt form. Overall, in the work conducted, the produced parts were found to be lighter than currently used parts and their mechanical values were quite close to the desired values. The shaping problems encountered in the sandwich composite produced at the project's beginning were not seen in the felt structure. Furthermore, since polymer and natural fibers are mixed within each other in felts, mechanical adhesion was observed and the need for the polymer to wet the fiber was reduced. Mechanical strengths in such structures can vary depending on the flow direction during production. In conclusion, vehicle lightweighting targeted in this work has been achieved, and in addition, a material with a significantly low carbon footprint has been produced. Work is ongoing on ensuring the part complies with automotive OEM specifications. These works include mechanical value testing as well as numerous climatic cycles and road tests. Furthermore, considering current conditions and requirements, the raw material cost of the part is found to be considerably higher than the current part. However, considering the regulations that will take effect in the near future and production volumes, it is thought that it will reach a viable level.   Kadir Çatak Senior Research and Development Specialist Fompak Ambalaj ve Poliüretan San. Tic. A.Ş.  

Sources • Kütahya Fompak Interior Trim Production Facility Management • Assoc. Prof. Güralp Özkoç and Team – Kocaeli University • Hassan Grup – Şiteks Tekirdağ / Saray Facilities • www.biyoplastik.com