Water, air, oil: What PLA bioplastic can (not) filter well

In the fight against the pan­dem­ic, air fil­ters are on every­one’s lips. With fil­ter mate­r­i­al made of non­wo­ven fab­ric, they block the way back into rooms for aerosols con­tain­ing virus­es. But how can these devices not only pro­tect health, but also be oper­at­ed with fil­ter mate­r­i­al that is as envi­ron­men­tal­ly friend­ly as possible?

Under clear­ly defined con­di­tions, the bio­plas­tic poly­lac­tide (PLA), also known as poly­lac­tic acid, is suit­able for this pur­pose. This can be deduced from results obtained by researchers from the Zuse com­mu­ni­ty in the recent­ly com­plet­ed “BioFil­ter” research project. The key ques­tion for this and oth­er poten­tial appli­ca­tions of biofil­ters is: How do the spe­cial prop­er­ties of PLA affect fil­ter per­for­mance and dura­bil­i­ty? After all, PLA can have prac­ti­cal dis­ad­van­tages com­pared to its fos­sil-based com­peti­tors. The mate­r­i­al tends to be brit­tle and it does­n’t par­tic­u­lar­ly like high tem­per­a­tures beyond 60 degrees Cel­sius. As a bio­genic mate­r­i­al, poly­lac­tic acid is also poten­tial­ly more sus­cep­ti­ble to wear and organ­ic degra­da­tion. This can play an even greater role in the use of fil­ters, e.g. in sewage treat­ment plants, than in air fil­ters. Indus­tri­al cus­tomers, how­ev­er, nat­u­ral­ly want a durable, reli­able product.

From monofilament to nonwoven

Against this back­ground, the researchers inves­ti­gat­ed PLA prop­er­ties in order to test non­wo­vens for biofil­ters on this basis. The Ger­man Tex­tile Research Cen­ter North-West (DTNW) and the Sax­on Tex­tile Research Insti­tute (STFI), where the non­wo­vens were pro­duced, were involved. Gran­ules from var­i­ous com­mer­cial­ly avail­able man­u­fac­tur­ers were used. How­ev­er, the inves­ti­ga­tions did not start with non­wo­vens, in which the fibers are deposit­ed close togeth­er in dif­fer­ent lay­ers, but with so-called monofil­a­ments, i.e. fibers made of PLA that are com­pa­ra­ble to threads. DTNW and STFI first car­ried out tests on these monofil­a­ments, e.g. in a cli­mate cham­ber for aging and durability.

As can be seen in the pic­ture, the monofil­a­ments became brit­tle after only two weeks at high­er tem­per­a­tures start­ing at 70 degrees Cel­sius, which the DTNW authors recent­ly report­ed on in the jour­nal Applied Poly­mer Mate­ri­als. Under stan­dard con­di­tions, how­ev­er, the monofil­a­ments showed no mea­sur­able reduc­tion in sta­bil­i­ty, even after almost three years, and the PLA non­wo­vens were also in no way infe­ri­or to their fos­sil-based coun­ter­parts in terms of fil­tra­tion per­for­mance. “The focus for the use of PLA as a fil­ter mate­r­i­al will, in my opin­ion, be on appli­ca­tions where rel­a­tive­ly low tem­per­a­tures are present, with which PLA copes very well,” says DTNW sci­en­tist Christi­na Schippers.

Considering other factors besides temperature and humidity

For the researchers, how­ev­er, the project, which was fund­ed by the Ger­man Fed­er­al Min­istry of Eco­nom­ics and Tech­nol­o­gy, was not just about the suit­abil­i­ty of poly­lac­tide for air fil­ters, but also for oth­er envi­ron­ments, such as fil­ter­ing water. In addi­tion, the research revealed that when eval­u­at­ing fil­ter media made from biobased and biodegrad­able non­wo­vens, it is impor­tant to con­sid­er oth­er influ­enc­ing fac­tors, such as mechan­i­cal stress­es caused by air cur­rents, in addi­tion to tem­per­a­ture and humid­i­ty. “The inno­v­a­tive core of the project was to eval­u­ate the pos­si­bil­i­ties and appli­ca­tion lim­its of PLA non­wo­vens as fil­ter media with suf­fi­cient mechan­i­cal prop­er­ties and long-term sta­bil­i­ty,” says project leader Dr. Lar­isa Tsarko­va. Like her col­leagues at STFI, DTNW is involved in the Zuse Com­mu­ni­ty’s Bioe­con­o­my Clus­ter, in which researchers from non­prof­it insti­tutes coop­er­ate under the guid­ing prin­ci­ple of “Research­ing with Nature.” “For us, the bioe­con­o­my is a top cross-indus­try top­ic that con­nects numer­ous insti­tutes of the Zuse com­mu­ni­ty and is lived through coop­er­a­tions such as with the ‘Bio-Fil­ter’,” explains the future STFI man­ag­ing direc­tor Dr. Heike Illing-Günther.

Cooperation in the bioeconomy cluster

With the results achieved from the “BioFil­ter” project, DTNW and STFI now want to con­tin­ue work­ing in order to be able to make deduc­tions for clear­ly described areas of appli­ca­tion for PLA non­wo­ven fil­ters in the future. These pos­si­ble fields of appli­ca­tion extend far beyond room air fil­ters and thus beyond the pan­dem­ic. For exam­ple, the water-repel­lent prop­er­ty of PLA is poten­tial­ly inter­est­ing for fil­ters in com­mer­cial kitchens for water-oil fil­tra­tion or in indus­try for engine oils.