Zlep�en� odolnos� proti horeniu v�aka radia�n�mu zosie�ovaniu bioplastov

Bioplastics: exploiting the potential

Bioplastics, which are obtained from renewable raw materials, reduce dependence on fossil resources. In addition, their use is usually associated with a better CO₂ balance than that of plastics of petrochemical origin. Some bio-based polymers are biodegradable and compostable. More durable bioplastics can usually be recycled and reused multiple times in line with the circular economy. 

Fig. 1: Radiation crosslinked, flame retardant PA 11 without (left) and with (right) wood particles after the UL 94 test. ©Fraunhofer WKI

The industry's demand for bio-based plastics is increasing. Europe has set the regulatory course to become climate-neutral by 2050, and consumers value sustainability. This is no longer just about packaging and comparable plastic applications, but also about technical materials with demanding property profiles. For example, plastics for applications in electrical engineering and electronics must be thermally resilient and flame retardant.  

Mechanism of radiation crosslinking

Many types of petrochemical-based plastics are known to withstand conditions after radiation crosslinking that they would otherwise not be able to withstand. High-energy beta or gamma radiation homolytically splits chemical bonds in the polymer molecules. This creates unstable fragments with a single electron, i.e. free radicals. As neighboring free radicals react with each other, crosslinks are formed: This creates a three-dimensional, very stable network.

Irradiation makes the plastics mechanically stronger, more heat-resistant, more abrasion-resistant and more resistant to chemicals.

Upgrading of biopolyamides

The petrochemical-based polymers that are suitable for radiation crosslinking include polyethylene (PE) and polyamides (PA). Alternatives are now commercially available for these plastics that are chemically identical or almost identical, but are produced from renewable raw materials. Therefore processors can directly replaceconventional plastics with these bioplastics without having to adapt their processes (drop-in bioplastics). Biopolyamides in particular are an attractive alternative for technically demanding applications. 

The company BGS Beta-Gamma-Service (Fig.1) has therefore investigated the effect of radiation crosslinking in particular on biopolyamides (PA 4.10 /PA 6.10/ PA 10.10 and PA 11) in detail. Experts compared the thermal behavior of untreated and crosslinked bio-PA by measuring the penetration depth of a soldering iron tip heated to 350 °C into test specimens. The result (Fig. 2): While the tip completely penetrates the test specimen made of non-crosslinked bio-PA within 5 seconds, it only reaches 0.2 to 1 mm with the crosslinked bio-PA types. The glass transition temperatures of non-crosslinked and crosslinked polyamides also differ significantly. With cross-linked bio-PA 10.10, for example, it is over 67 °C, around 10 °C higher than with untreated PA.

Fig. 2: Penetration depths of a 350 °C soldering iron tip in radiation crosslinked test specimens made of various biopolyamides. The tip penetrates almost unhindered into non-crosslinked samples. ©BGS

The dynamic mechanical analysis (DMA) also shows the effect of radiation crosslinking (Fig. 3). The storage and loss modulus of non-crosslinked bio-PA 6.10 drops sharply at around 225 °C: The material begins to melt completely. Radiation crosslinked PA 6.10 is different: It retains a residual stiffness above the melting temperature. In the amorphous areas of the plastic, the crosslinking points hold the polymer chainstogether. Components made from the crosslinked bioplastics therefore exhibit sufficient strength even at temperatures at which their untreated counterparts would fail. Conclusion: Biopolyamides can be radiation crosslinked, which significantly improves their heat resistance, among other things.

Fig.3: The dynamic mechanical analysis reveals that in contrast to untreated test specimens, radiation crosslinked bio-PA 6.10 retains a residual stiffness even above 230 °C. ©BGS

Flame retardant biopolyamides

The radiation crosslinking of bio-PA thus opens up the possibility of meeting the demanding thermal requirements of products in the electronics and electrical engineering industries, for example. However, such products must also meet strict flame retardancy requirements. The Fraunhofer Institute WKI and BGS and other industrial partners have therefore jointly developed formulations for injection molding that are based on bio-PA and equipped with flame retardants. They also investigated the crosslinkability of these formulations. The project was funded by the Federal Ministry of Food and Agriculture via the Agency for Renewable Resources.     

The partners carried out glow wire tests and measurements of tracking resistance. Selected flame retardant formulations based on bio-PA met the target requirements (glow wire test: 960 °C, tracking resistance: CTI value 600 V). The addition of wood particles improved the flame retardant performance. The project partners proved that the flame retardant bio-PA grades are radiation crosslinkable, albeit to varying degrees. The UL 94 flammability test showed that the classification was obtained after radiation crosslinking when wood particles were used (Fig. 1). Flame retardant bio-PA 6.10 showed the best crosslinkability, while bio-PA 11 showed lower degrees of crosslinking under comparable irradiation conditions. As expected, radiation crosslinking increased the tensile strength (Fig. 4) and the tensile modulus of elasticity, while the notched impact strength decreased. Based on these findings, there is great potential for developing customized formulations for the respective application that achieve the properties of flame retardant and radiation crosslinkable PA 66, for example.

Fig.4: The tensile strength of flame retardant bio-PA test specimens can be improved through radiation crosslinking. ©Fraunhofer WKI

Tests with polylactide (PLA)

In addition to bio-based drop-in polymers, there are also bioplastics based on renewable raw materials. These include polylactic acid (PLA), which has its origins in plant starch. The research consortium led by the Fraunhofer IAP also gained new insights into the radiation crosslinking of this bioplastic. The project partners extensively tested additives to achieve radiation crosslinking of PLA. They found an additive that has a positive effect on the material when used with electron irradiation. The crosslinking of the polymer molecules demonstrably outweighs the degradation of PLA caused by the radiation. In the synthesis of halogen-free, novel flame retardants based on bio-based alcohols and phosphorus-containing compounds, the focus on the production of fully esterified phosphates proved to be promising. After optimization, compounding with PLA was able to be achieved. Flammability tests in accordance with UL94 resulted in a very good classification (V-0) with a test specimen thickness of 1.6 mm. 

New application possibilities

The research work shows: There are many parallels to conventional plastics in the production, processing and radiation crosslinking of bioplastics. Flame retardant finishes and radiation crosslinking open up new application possibilities for bioplastics in electrical engineering and electronics, but also in the automotive industry, for example.    

Learn more about radiation crosslinking.