Tsarkova: The advantage is that PLA is a bio-based, biodegradable, and biocompatible polymer, which is why it is widely used in medical applications such as surgical sutures or implants. The biggest problem pertains to its mechanical properties: The material tends to be brittle and is subject to hydrolytic degradation at high temperatures beyond 60 degrees Celsius. Implants are a great way to illustrate the lifespan and the subsequent effects. Implants are designed to fulfill certain functions, meaning they are meant to mechanically support bone or tissue growth temporarily. In this case, we can take advantage of the degradable properties of PLA. That being said, things become problematic if the material becomes brittle and loses its stability before the allotted time is over. That would mean the implant breaks before the bones have coalesced. When you use PLA, the challenge is to shape the mechanical properties in a way that maintains them throughout the product lifecycle. Ideally, the product is then biodegraded afterwards.
Our work has studied PLA nonwovens and PLA monofilaments, which are used as a surgical suture material, for example. We discovered that the disadvantage of polylactide brittleness can be overcome with specific manufacturing processes, thus ensuring that the PLA fibers are strong and ductile, yet still feature biodegradability as an attractive property. The key finding of the research is that the manufacturing process could be used to program the material to remain stable and solid for a predetermined period to fulfill its purpose and to slowly break down into CO2 without leaving traces of microplastics when it is no longer needed. Our next step is to transfer this principle to nonwoven fabrics that can subsequently be used in wounds pads, for example.
What is the filtration performance of PLA and what are its filtration applications?
Tsarkova: The major drawback of PLA fabric is that it cannot be used for temperatures beyond 60 degrees Celsius as this is close to its glass transition temperature (the polymer softens at that point). That’s uneconomic from a business perspective because filter manufacturers usually have their products certified up to 70 ° C as this meets the common standard. However, many uses of filtration don’t exceed temperatures of 40-50 ° Celsius. That is why PLA filters would not see a widespread use in the automotive industry. Yet the filters could be used for medical and biotechnical filtration, water treatment systems or to produce nonwovens and melt blown face mask filter material.
Taubner: The filtration performance of PLA keeps up with its counterparts. It can be used as a filter material to catch aerosols in the air, for example in FFP2 masks (equivalent to other international standards known as N95, KN95 and P2 masks).
Is a PLA filter as durable as a conventional fossil-based filter?
Tsarkova: There is no risk when the filter is stored under normal temperature and humidity conditions. Even after three years of storage, we detected no changes in mechanical properties. Having said that, the higher the temperature and humidity, the faster PLA breaks down. The filters disintegrate within a few weeks at temperatures beyond 60 ° C and a relative humidity of over 80 percent.
Taubner: Those are actually the conditions in industrial composting. The goal of the latter is for the filters to decompose, making them areas where PLA is no longer applicable.
Apart from air filter systems, can the filters also be used in medical applications?
Tsarkova: I think the membrane filters are suitable for all applications that don’t exceed critical temperatures. Water is also not a problem since the product is hydrophobic. Unless the filter is being used for several years – and that is definitely not the case with air filters or medical-grade masks – it can be used for these purposes.