Dynamic Strength Properties of Biopolymers based on PLA, PBS, PBAT, and TPS for Energy Absorbing Structures

The growing awareness of society and the high cost of fossil fuels result in a continuous increase in the popularity of alternative means of transport (e.g. bicycles). There is a great, continuously increasing demand for personal protective equipment, such as sports helmets. The energy-absorbing layer of all currently produced helmets is made of foamed plastics: polystyrene (EPS), polypropylene (EPP), or polyurethane (EPU). According to the manufacturers' recommendations, these materials should be immediately replaced after each collision. <b>This procedure is often skipped because helmet damage is not visible to the naked eye, and the users are aware of the material value of their helmets. The solution may be changing only the energy-absorbing layer, as the helmet's exterior shell is primarily intact.</b><br/>The authors want to present environmentally friendly protective honeycomb inserts made of biodegradable polymers invented during project implementation. Thanks to their structure, they can constitute a reusable alternative to polystyrene (the insert in the shell can be replaced each time after an accident).<br/>The authors of the manuscript <b>created unique blends of biodegradable plastics based on PLA</b> with the addition of bioplastics, such as polybutylene succinate - PBS, polybutylene adipate coterephthalate - PBAT, and thermoplastic starch - TPS. They combine high strength and plasticity, and thanks to their biodegradability, these mixtures meet the criteria of sustainable development. <b>All developed materials were extensively tested using a unique method developed by the authors, which relies on dynamic tensile tests. This brought the results closer to conditions occurring during actual incidents (e.g. collisions).</b><br/>Properly designed geometry of the energy-absorbing structure in combination with mechanical properties (tensile strength, yield strength, elongation, Young's modulus) tailored to specific geometry and applications <b>resulted in the invention of the thin-walled biodegradable structure that engages the most effective mechanism, which is dynamic, plastic folding.</b><br/><b>There are no standards describing strength tests conducted in dynamic conditions. The conducted tests constitute the "know-how" of the authors</b>. The testing device was based on the design of a rotary hammer and was equipped with a flywheel with a diameter of 0.6 m and a weight of about 230 kg. The flywheel's rotational speed was preset to reach the strain rate from 250 to 1000 s^-1, which is close to the actual condition during the crash. A specially designed measurement system allowed to reach the sampling rate equal to 1 MHz. In order to accurately measure the deformation, the process was additionally filmed with a high-speed camera capable of registering 1,000,000 fps.<br/>The criterion of suitability and safety of a given material for a designed geometry of insert were strength properties determined in dynamic tensile tests (Re &gt; 15 MPa, Rm &gt; 10 MPa, A &gt; 10%).<br/><b>Tests confirmed that complex thin-walled honeycombs can be successfully molded and used as energy-absorbing structures</b>. On this basis, it can be concluded that the <b>developed biodegradable blends provide the desired compromise between mechanical properties - high tensile strength and satisfactory plastic properties engaging plastic folding mechanism during the deformation</b>. These biomaterials can also be used in other energy-absorbing applications. Although the developed blends are biodegradable, they are sufficiently resistant to variable weather conditions (e.g. sunlight, rain exposure) to fulfill their purpose during the exploitation time. The developed materials and testing methodology will affect personal protection equipment and shows potential for applications in the automotive, food, and packaging industries.<br/>Funding: This work was supported by the project BIOKASK "Development of innovative, replaceable, energy-absorbing structures based on biodegradable plastics for protective helmets" (0223/L-11/2019, LIDER, NCBR).